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Heat the oven to 180C/160C fan/ gas 4. Butter the base and sides of two deep 20cm cake tins, and line with a double layer of baking parchment. Beat the butter and sugar in a stand mixer or using an electric whisk until pale. Slowly add the egg and a spoonful of the flour if it starts to split. Sift in the rest of the flour, then add the vanilla. Fold in using a large metal spoon. Divide between the tins and bake for 50-55 mins until a skewer inserted into the centres comes out clean. Cool in the tins for 15 mins, then invert on wire racks to cool fully.
Using a serrated knife, cut each cake in half through the equator. Level the tops. Put one cake layer on a board or stand, spread with a third of the curd, then repeat twice, sandwiching with the final cake.
For the buttercream, beat the butter in a stand mixer or using an electric whisk (see tip, below). Add the icing sugar in three additions and whisk until fully combined. Add 1-2 tbsp water until smooth and spreadable. Spread a thin layer of buttercream over the top and side of the cake using a palette knife. Chill for 30 mins until firm.
Meanwhile, melt the white chocolate in a heatproof bowl over a pan of simmering water, or in the microwave in short bursts. Cool slightly, then coat the outsides of the waffle cones in it. Pour sprinkles into a shallow plate and roll one of the cones in them to coat fully. Put all three on a tray lined with baking parchment and chill until set.
Use a palette knife to smooth most of the remaining buttercream over the cake. Divide the rest between small bowls and dye with the food colouring gels of your choice. Spoon into piping bags fitted with different piping nozzles. Use the fondant icing to shape the eyes, nose and mouth of the animal, and attach to the front of the cake. Pipe dots and swirls of buttercream on top to make hair. Attach the plain waffle cones to the side (pointy ends inwards) to make ears, and top with the sprinkle-coated come for a hat. Decorate with edible flowers and candles, if you like. Will keep chilled for up to three days.