Chocolate Sheet Cake
6 ounces bittersweet chocolate, chopped fine
2 ounces Dutch-processed cocoa powder
12 ounces boiling water
7 ½ ounces unbleached all purpose flour
10 1/2 ounces granulated sugar
1 tsp table salt
1 tsp baking soda
6 fluid ounces vegetable oil
1 tablespoon white vinegar
2 teaspoons vanilla extract
Adjust oven rack to middle position and heat oven to 350 degrees. Line one 17 by 13 half sheet pan with parchment paper and spray with baking spray. Place chocolate and cocoa in medium bowl. Pour hot water over mixture and whisk until smooth. Set in refrigerator to cool completely, about 20 minutes. Whisk flour, sugar, salt, and baking soda together in medium bowl; set aside.
Whisk oil, eggs, vinegar, and vanilla into cooled chocolate-cocoa mixture until smooth. Add flour mixture and whisk until smooth.
Spread batter evenly in the pan. Bake until cake is set and just firm to touch, 15-20 minutes. Cool cake in pans until room temperature. Carefully remove cake from pans and use a 4 inch round cookie cutter to cut 24 rounds. Stack and sandwich with buttercream.