Melt the chocolate and butter together in a double boiler.
Mix the eggs and sugar, then whisk until foamy.
Add the melted chocolate and butter to the egg mixture and mix well.
Sift cake flour and cocoa powder into the mixture and mix until evenly combined.
Pour the batter into a pan lined with parchment paper.
Bake in a preheated oven at 180°C for 20 minutes.
Let it cool, then cut into bite-sized pieces.