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