vegetable-oil cooking spray
4 sticks unsalted butter, cut into pieces
8 oz unsweetened chocolate, chopped
1 3/4 c all-purpose flour
1/2 c unsweetened cocoa powder (preferably Dutch-process)
1/2 t salt
3 3/4 c granulated sugar
8 large eggs
Preheat oven to 350 degrees and line mini-cupcake tins with liners. Spray liners with cooking spray. If not using liners, mix 1 T cocoa powder with 1/2 c flour, spray tins with cooking spray and dust cups thoroughly with the cocoa-flour mixture.
Melt butter and chocolate in a 4-qt heavy pot over moderately low heat, stirring until smooth. Whisk together flour, cocoa and salt. Remove pan from heat and whisk in granulated sugar. Add eggs, 1 at a time, whisking after each addition until incorporated, and stir in flour mixture until blended.
Spoon batter into muffin liners, filling cups to top, and bake in middle of the oven 25-30 minutes, or until a tester comes out with crumbs adhering. Cool 5 minutes in tins and turn out onto racks. Repeat with remaining batter.