Preheat oven to 400°. Line the bottom of a 13x9x2" metal baking pan with
parchment paper. Butter paper and sides of pan; dust with cocoa powder, tapping
out excess. Sift flour and remaining 1/4 cup cocoa powder through a fine-mesh
sieve into a small bowl. Repeat sifting 2 more times. Set flour mixture aside.
Crack 2 ...