Donnie wasn't a big laugher. He didn't have quite the stick up his butt Leo did (and probably still does, not that Mikey would know) but he was still prone to losing himself in his work and missing out on punchlines and one-liners, even the really good ones.
Mikey never really begrudged for it, not when he knew how much Donnie did for them, and fuck knew he wasn't going to go after one for being lost in thought. Still. Don's absentmindedness had always presented a challenge, and Mikey loved himself a challenge.
Anyway, that was the reason he gave for saving all of his greatest hits for Donnie, his most daring feats of slapstick and wordplay and all that cool stuff from the comedy books. It took a while for him to admit to himself that it might have also had something to do with the way Donnie's eyes lit up when he laughed, and how the sight always made Mikey breathe a bit more easily.
Donnie didn't always laugh, of course--it wouldn't be a challenge if he did. But when he did laugh, soft and bright, almost shocked by the sound of his voice transformed by joy...Mikey loved it. He loved it so much it hurt and he pulled that hurt close to his chest, savoring it.
Now, it hurts more than it ever before, but he still loves it. Even when he's sitting in the dark with his stump aching and a gun propped up at his side, eye closed against the nothingness--even then, he can love his brother's laugh.
"--and the whole room was full of pasta!" he finishes, waving his remaining arm with a triumphant swish.
Nearby, Donnie chuckles, the sound ringing in Mikey's ears. He laughs more easily these days, but it still gives Mikey that little rush of victory to hear it.
"You woulda liked that, huh, Don," he muses, spreading out his arms. "A room full of spaghetti would've kept Donaconda happy for days. Damn, but you could pack it away, couldn't you?"
Another chuckle, quieter this time. He feels the faintest weight on his shoulder, unless it's just the press of his jacket.
"Everybody thinks I'm the big eater in the family," he says. Thought, his mind whispers, but Mikey tries to ignore it. "That's ‘cause I made the biggest show and all that, y'know. Ringmaster Mikey, at your service." He gives a mock bow, all dramatic.
"But you, you, just sat yourself down and kept on chewing, nice and steady, until it was all gone, and then you'd go beg me for seconds. And I'd hand 'em over, because you had the biggest puppy dog ever, even if you only whipped 'em out for me. No one ever believes me, but I knew." He's faintly aware that he's switching tenses again, a possible bad sign, but right now he's too tired to care.
"And then you'd--" He cuts himself off at the soft press of lips against his skin, dragging over his flesh. His eyes stay tightly, firmly shut.
"Donnie?" he asks. "You hungry?" Silence, but he can feel the faint press of a tongue against his skin. "Donaconda strikes again, huh?" A chuff of laughter, rippling over his flesh.
"Yeah, I thought so." He keeps his voice light while trying not to move his mouth too much, to not disturb the trail of kisses he can feel climbing along his neck and up his face. "You can't keep away from me, brah. I'm on the hottest piece of meat on the streets."
Laughter against lips, warm against his tongue. He sighs into Donnie's kiss, leaning forward a second before realizing that Don's pulling away. Cool, callused hands gently push him down and Mikey slumps back, feeling his brother's tongue and fingers drift gently down his plastron.
(He doesn't know whether it's a ghost or hallucination that's doing this to him, truly, and Donnie never asks his questions. Mikey's considering eating a bullet to find out, but that wouldn't make Donnie laugh at all, and Mikey hates that).
"I ever tell you about the, hnnh, rabbit who fell in love with a monkey?" he asks. "There--fuck--" He feels the warm press of a mouth against his slit, sharp and insistent. "The rabbit's called Carritron, y'know, and the monkey's Mr. Bananas, and together they..." Laughter right up against his slip, ripping through him. "Gah! Shit!"
He spreads his legs wider, letting his cock spill free. Donnie catches up with a quick mouth and able fingers, heat rushing over him like a balm. Laughter flexes through Donnie's throat, silently buzzing through Mikey's skin.
"Or, maybe..." He squirms into his brother's mouth, shuddering all over. "Uh, you ever, ever heard the one about the plumber and the, the baker? Like the plumber s-shows up at the bakery and uh, mmh--" 
Mikey lets out a sharp gasp as he feels Donnie's mouth abruptly pull away, letting cool air tease Mikey's swelling cock. "C'mon, man," he begs, half-playful and half-desperate hips wriggling frantically.
Donnie laughs at him, at the sight he must make, the sound seeping in through Mikey's pores and undoing him from the inside out. He groans, and it feels a bit like a sob. Tears trickle against his cheeks and he feels his brother's lips come to brush them away.
Then tight, warm heat coming down, down over his cock. It feels like the first time--the only time--so good Mikey lets out a frantic squeak. Donnie laughs at the sound, wriggling his hips like a fucking demon or angel or something, and it's everything Mikey can do not to pass out from joy.
He feels himself pushed down, on his back, concrete cruel against his flesh. His brother's fingers brush over his face, precut-slick and quietly insistent.
"You want more?" Mikey asks. He's trying to sound cocky, but the words bend and shake as Donnie starts to ride, all that fire and power and beauty bearing like stars falling from the dark. "I, fuck, I'll--I'll give you, more."
And he does, words spilling from him in a sharp, clotted tangle, snappy lines with no context and punchlines with no builds, funny names for creatures they haven't seen in years, fragments of half-remember stories and tall tales that fall apart as soon as he tries to focus on them for too long. It's all he has left of his old self, his old wit, his comedian's panache that used to make him unstoppable.
He gives it to Don, all to Don, handed up like an offering. And Don answers with laughing, and laughing, laughing with the same pleasure that makes Mikey moan and gasp and whimper. Laughing in a way that shouldn't make no sense, but it's still his brother's voice, his brother's song, vibrations pushing through his cock and into Mikey's core.
When Mikey comes, Don bends down and presses their lips together, spilling laughter down Mikey's throat like a current of fiery, like a blessing. Mikey screams Donnie's name back into his mouth, because it may not be a joke, but it's still the lightest and most joyful thing he's ever held on his tongue.
Everything sorts of melts after that, a blaze of stickiness and heat and buzzing afterglow. He thinks Donnie might have come at some point, although it's hard to tell. He thinks he feels Don laying him down and wrapping blankets around him, tucking him close, but it's hard to tell.
He'll probably wake up tomorrow and find himself alone, with only the faint smell of his own cum in the air, but it's hard to tell. Donnie snuggles up against him and Mikey takes his hand, rubbing his fingers over his brother's hand, savoring every callus.
"I love you," he whispers, and that one's not a joke either.
Donnie laughs, but not a mocking laugh. It's an accepting laugh, a loving laugh, a tired laugh, maybe even a said laugh. It's a laugh that says I know and me too and I'm sorry and go to sleep, dumbass. 
So Mikey closes his eyes and goes to sleep, feeling the real dark, the dark inside his head, tug him away. Donnie's echoes follow him all the way down.