Heat oven to 180C/160C fan/gas 4. Put 12 cases in a 12-hole cupcake tin. Whisk the eggs and sugar together until light and fluffy – this will take about 5-8 mins with a stand mixer or 10 mins using an electric hand whisk. The mixture will have increased in volume and should be thick enough to leave a trail when the whisk blades are lifted.
Gently fold in the melted butter. Sift the flour, cocoa and baking powder over the surface and fold in using a large metal spoon, trying not to knock out too much air. Divide the mixture between the cases, then bake for 15-20 mins until well risen and the sponge springs back when gently pressed. Leave to cool in the tins for 5 mins, then lift out onto a rack.
To decorate, microwave the chocolate on medium for 20-30 secs, stirring every 5 secs, until melted. Gently stir until smooth. Spoon 12 'puddles' of chocolate onto a piece of foil. Divide the chocolate eggs and bunnies between the puddles, to decorate. Leave to set in the fridge or in a cool place.
Beat the butter, icing sugar, vanilla and and 2 tsp boiling water together using an electric whisk for 4-5 mins until light, pale, fluffy and nearly doubled in size. Spoon into a piping bag with a star nozzle. Pipe onto the cupcakes, then stick the chocolate puddles on top.