1Pre-heat the oven to 180C/160C Fan/Gas 4. Grease an ovenproof 1L pudding basin. Line the base with baking paper.
2Put 115g butter, 115g caster sugar, 2 eggs, 115g flour and 1/2 tsp baking powder in a bowl. Beat for 2-3 minutes with an electric hand-whisk, until pale and fluffy. Set aside.
3In another bowl, beat the 60g butter, 60g sugar, 1 egg, 40g flour and 1/2 level tsp baking powder, plus 25g cocoa powder and 1 tbsp milk.
4Put alternate spoonfuls of the two mixtures in the pudding basin. Level the surface.
5Repeat the entire process again in order to make 2 bowl-shaped cakes.
6Bake for 45-55 minutes, until risen. To check if the cakes are ready, insert a skewer into the centre; it should come out clean.
7Cool in the basin on a wire rack for 15 minutes. Turn out and leave to cool completely.
8Dissolve the granulated sugar in 125ml water in a pan. Heat to 108C, or until a fork dipped in it brings away strands of liquid sugar.
9Lightly whisk the egg yolks with an electric whisk. Pour the sugar syrup down the side of the bowl in a thin stream, whisking all the time, until mousse-like.
10In a separate bowl, beat the butter until soft, then add to the egg mixture, 1 tsp at a time. Add the vanilla. Chill for 45 minutes.
11Cut a 2cm-thick slice of the fat top off one of the sponges. Spread icing on the cut-off layer, then stand the rest of the cake on top, rounded-side down, to form an egg cup shape. Cover with a thin layer of icing.
12Using the fat side of a knife, press chocolate strands all over the outside of the egg cup. Decorate with sugar flowers. Stand on a cake board.
13Turn the other sponge cake rounded-side up. Cover with butter icing, then smooth on the ready to roll white icing. Trim around the bottom, then sit the iced cake on top of the egg cup. Tie a ribbon around the join.
14Cut a hole in the top of the cake and fill with mini eggs. Add custard yolk to serve.