Preheat the oven to 180°C (356°F).
Melt the dark chocolate and butter together using a double boiler.
In a bowl, combine eggs and sweetener and mix well.
Add the melted chocolate and butter, then stir well.
Sift cake flour and cocoa powder into the mixture and mix thoroughly.
Pour the batter onto a baking pan and spread evenly.
Bake in the preheated oven at 180°C for 20-25 minutes.
Let the brownie cool, then cut into bite-sized pieces.