Green’s Dictionary of Slang

(orig. US black) excellent, doing very well, esp. of a performance.

1962		(con. 1950s) H. Simmons Man Walking On Eggshells 222: No kidding, man; his group’s cooking, huh?
1978		(con. WWII) T. Sanchez Hollywoodland (1981) 86: The Hollywood Stars were cooking. Angel was breathing red hot and throwing comets.
1983		G. Tate ‘King Sunny Adé’ in Flyboy in the Buttermilk (1992) 59: Even when these cats ain’t but half-cooking, there is no way you can see them [...] and not be rocked to your knees.
2002		D. Flusfeder Gift 187: We’re great, Bud. Cooking.
2005		L. Theroux Call of the Weird (2006) 51: We were flying. We were cooking.
2012		S. Blass Pirate for Life 97: When a pitcher is on a roll [...] you’re so confident that you often think, Okay, you want that one? I can get him out with that one, too. That’s when you’re really cooking.