Finely chop the dark chocolate.
Melt unsalted butter and dark chocolate together in a double boiler.
Mix eggs and erythritol until well combined.
Combine both mixtures and add vanilla extract.
Sift in cocoa powder and almond flour, then mix until blended.
Pour the batter into a baking tray.
Bake in a preheated oven at 180°C for 20 minutes.