Add the unsweetened chocolate and bittersweet chocolate to a heavy-bottomed saucepan and place over low heat. Stir constantly until melted. Let cool.Note: The more you have it chopped up, the quicker it will melt.
Whisk eggs in a large bowl. Add sugar and whisk for 1 minute.
Whisk in vanilla and then melted butter.
Whisk in the melted chocolate.
Place all-purpose flour, cake flour, cocoa powder, baking powder, and salt in a sifter or fine-meshed sieve. Sift into the batter bowl. Stir the flour mixture into the chocolate mixture until no flour is visible.
Stir the chocolate chips into the batter and transfer the batter to a greased 9-inch square baking dish.
Bake for 30 to 32 minutes. Let cool 2 to 3 hours before slicing.
If you don't have superfine sugar, you can porcess regular granulated sugar in a food processor to turn it into superfine sugar. Be sure to measure the sugar AFTER you process it. It measures very differently since the granules are smaller.
This recipe originally appeared on spicysouthernkitchen.com