Melt the bittersweet chocolate and butter together using a double boiler.
Add sugar to the melted chocolate mixture and mix well.
Add eggs one at a time, mixing thoroughly after each addition.
Sift in cake flour and cocoa powder, then combine until just mixed.
Pour the batter into a pan and smooth the top evenly.
Bake in a preheated oven at 170℃ for 25-30 minutes.
Let it cool, then cut into bite-sized pieces.