Melt dark chocolate and butter together in a double boiler.
Add eggs and erythritol to a bowl and mix well.
Add the melted chocolate mixture to the egg mixture and mix until combined.
Sift in the almond flour and cocoa powder, then mix until evenly combined.
Line a baking pan with parchment paper and pour in the batter.
Bake at 180°C (356°F) for 20 minutes in a preheated oven.
Let it cool completely, then cut into desired serving sizes.