Heat the oven to 180C/160C fan/gas 4. Butter the sides of a 25 x 35cm baking tray and line the base. Melt the rest of the butter in a small saucepan and take off the heat. Mix the milk, yogurt, vanilla and eggs and stir in the butter.
Tip the dry ingredients plus ¼ tsp salt into a large bowl. Stir through with a whisk and then pour in the wet ingredients, whisking gently as you do so. Keep whisking to a smooth, silky batter. The batter will start to make bubbles straight away so pour it into the tin and shake it to level the top. Bake for 20-25 mins or until risen and golden, and remove from the oven. Cool for 10 mins in the tin, then carefully invert the cake onto a cooling rack.
Make the buttercream by tipping the butter into a large bowl and sift the icing sugar on top. Add the vanilla and milk and a pinch of salt then beat carefully for a few mins with an electric whisk until it is creamy, pale and spreadable. If the icing is too stiff add a little more milk.
Invert the sponge onto a serving board. Cut a bunny head from the sponge using a sharp knife. Cut a nose with a round cutter from the excess, cut the round in half and cut a small oblong to fit between the semi circles to make an oval. Use a little buttercream to stick the nose onto the sponge, ice the outside of the nose with a layer of buttercream. Scoop ¾ of the buttercream into a piping bag fitted with a star nozzle.
Pipe rows of buttercream around the outside of the cake and all over the face. Make a row of buttercream stars around the ears leaving some space in the middle for a different colour. Colour the remaining ¼ of icing with pink and pipe the centre of the ears. Make a nose shape and two eyes from the fondant and stick them on the face. Paint the nose pink and the eyes blue and black. This cake will keep for 3-4 days somewhere cool. Freeze the offcuts to use in a trifle.