Divide the combination between the tins and clean the highest with the again of a spoon.
Bake for 20-25 minutes till springy to the contact. Take out of the oven and depart within the tins for 10 minutes earlier than turning out onto a wire rack to chill utterly.
In the meantime, make the buttercream by melting 50g darkish or milk chocolate within the microwave, or in a bowl over simply simmering water, ensuring the bottom doesn’t contact the water. Depart to chill.
Beat 85g softened unsalted butter and half of the 175g sieved icing sugar collectively till mild and fluffy. Add the remaining icing sugar and melted chocolate and blend collectively. If the icing is runny, chill within the fridge till it’s agency however nonetheless spreadable.
To assemble the cake, put a small dollop of icing onto a plate and put one of many muffins on prime. Unfold the highest of the cake with half the icing, leaving a 1cm border across the edge.
Put the second cake on prime and push down very gently. Unfold the remaining icing excessive.