Preheat the oven to Gas 4, 180°C, fan160°C. Line a 20cm (8in) springform cake tin with nonstick baking paper.
Melt the chocolate and butter in a bowl resting over a pan of simmering water, making sure the base doesn't touch the water. Cool a little.
Whisk eggs and sugar together for about 10 minutes until light and foamy. Whisk in the cooled chocolate mixture, then whisk in the flour and cocoa.
Pour into the tin and bake for 40 minutes. Leave in the tin for 10 minutes, then remove and cool on a wire rack.
Melt the white chocolate cake covering in a bowl resting over a pan of simmering water, making sure the base doesn't touch the water. Pour into a small, clean tub and chill until set.
For the topping
Melt the white chocolate in the same way. Cool a little. Beat the butter and icing sugar in a bowl until soft and creamy. Beat in the milk using a wooden spoon, then fold in the cooled white chocolate.
Cut the cake through the middle horizontally and spread a third of the buttercream over the base. Replace the top and use the remaining icing to cover the top evenly.
Use a vegetable peeler to make curls from the set white chocolate cake covering. Sprinkle them around the cake edge and decorate the middle with mini eggs.
This cake freezes well. Once it's cool, wrap well in clingfilm and freeze for up to a month. Cool overnight at room temperature before serving
Some products in this recipe are seasonal and so may not be available to buy online all year.