Butter three 20cm round cake tins and line with non-stick baking paper. Heat the oven to 190C/170C fan/gas 5. Beat the butter and sugar together in a large bowl until light and smooth. Gradually beat in the eggs until combined, then mix in the vanilla. Fold in the flour and baking powder until everything is combined. Stir in the milk to loosen.
Bake the sponges for 20 mins until golden and a skewer inserted into the centre comes out clean. Leave to cool in the tins for 10 mins, then transfer onto a wire rack to cool completely.
Meanwhile, make the Swiss meringue buttercream. Put the egg whites and sugar into a heatproof bowl over a small pan of gently simmering water. Make sure the bowl doesn’t touch the water. Whisk for 3-4 mins until the sugar dissolves. Put into an electric mixer and whisk for 10 mins over a medium-high speed until you have a thick meringue.
Gradually add the butter into the meringue mixture for 5 mins until incorporated and the buttercream is fluffy.
Put one sponge onto a plate or cake stand, top it with a few tablespoons of the buttercream and spread into an even layer. Add the next sponge, and repeat. Top with the final sponge.
Smooth a thin layer of the buttercream around the cake, so all the sides are coated – this is called a crumb coat. Chill in the fridge for 45 mins. Remove and smooth a thick layer of the remaining buttercream around the cake using a palette knife, reserving a little for decoration. Put back in the fridge for 45 mins-1 hr to set.
Meanwhile, make the ganache. Put the dark chocolate in a bowl and heat the cream in a small pan until just steaming. Pour the cream over the chocolate and leave to melt for 5-10 mins. Stir to make a glossy ganache, then set aside to firm up a little – you want it to be pourable but not too runny. Spoon the ganache over the top edge of the cake, encouraging it to drizzle down the side with your spoon – start at the back of the cake to get the hang of it. Fill in the middle of the top of the cake with chocolate too. Leave to set for 10 mins.
To decorate, put the leftover buttercream in a piping bag with a round nozzle and pipe little dots on top of the cake. Decorate as you like, remove any flowers before eating.