Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line 2 x 22cm round cake tins.
In a mixing bowl, combine the cocoa powder with 200ml boiling water and stir until smooth. Sift the flour and baking powder into a separate mixing bowl, then add the vanilla extract, eggs, oil, sugar, melted butter and cocoa mixture. Whisk until smooth, then divide between the prepared cake tins. Bake for 40-45 minutes or until a skewer inserted into the centre of the cakes comes out clean. Remove from the oven and let cool in the tins for 10 minutes, then turn out on to a wire rack to cool completely.
Meanwhile, beat the butter in a large bowl until soft. Add half the icing sugar and beat with a handheld electric whisk until combined. Add the remaining icing sugar and beat until the mixture is smooth and creamy. Stir in the melted white chocolate and set aside.
Put 1 cake upside down on a serving board and thickly spread with the buttercream, then sprinkle over the crushed Mini Eggs. Top with the other cake and, using a palette knife, spread half the remaining buttercream over the top and sides – don’t worry if you can see crumbs poking through. Once the cake is covered, repeat with the remaining buttercream.
Stir the wholewheat biscuits into the melted dark chocolate until coated, then add spoons of the mixture to the top of the cake in a ring shape. Scatter over the chocolate shards and sprinkle over the extra shredded wheat strands. Position the egg in the centre of the ‘nest’ and press the whole and halved Mini Eggs around it. Finish off the cake with any other Easter decorations you wish.