"Whoa!" It's really only a chunk, and yet Mac levitates about half a foot when it hits the back of his head. Something about the whole, you know, everything of this place has his a little more high-strung than he usually is, and so when Dave comes at him like some kind of cake-pitching hooligan, Mac responds by losing his chill. Just a little*.

"What the fuck was that all about, you little bitch?" Mac swats the mess of cake from the back of of his head and rolls up on a literal teenager, as one does when they're a tough guy over twice said teenager's age. He actually fingerwags, which is bound to do quite a lot for his credibility. "If you're going to be throwing a food fight, it needs to be with food that's actually edible."

And he chucks a handful of stale donut at Dave's chest.

*Mac also enjoys food fights, which serves as an additional impetus.