Melt the dark chocolate and butter together using a double boiler.
In a bowl, mix the eggs and erythritol.
Add the melted chocolate and butter and mix well.
Sift in the almond flour and baking powder, then mix.
Slice the banana thinly and place it on top of the batter.
Pour the batter into a pan and bake in a preheated oven at 170°C for 20 minutes.