Melt the butter and dark chocolate by putting in a bowl over a saucepan of boiling water. When melted, remove from the heat and allow to cool to room temperature.
In a large bowl, whisk together the eggs and sugar until very pale and fluffy. Pour in the melted butter and chocolate and fold in gently.
Tip in the flour and cocoa powder and continue to fold until fully combined, but don’t over mix.
Pour into a 20cm cake tin lined with baking parchment. Put in the oven for 25 minutes.
Remove from the oven and, while still hot, arrange your Christmas chocolates on top. Transfer to the fridge and allow to cool completely before slicing into 12 equal pieces and serving.