50g each white, plain and milk chocolate, broken into squares
Heat oven to 180C/fan 160C/gas 4. Butter and line the base of a 18 x 28cm cake tin or tray. Put the butter and sugar into a bowl; beat together with electric hand beaters until light and fluffy. add the eggs a little at a time, beating well between each addition.
Sieve the flour, cocoa and bicarbonate of soda into the bowl, then tip in any bran left in the sieve. Pour in the natural yogurt. Stir everything to a smooth mixture and spoon into the prepared tin. Bake for 20-25 mins until just firm and shrinking away from the sides of the tin. Cool in the tin for 5 mins, then turn onto a wire rack to cool completely.
For the icing, sieve the icing sugar and cocoa into a bowl, then pour in the butter and 2 tbsp just-boiled water. Stir together to a smooth, spreadable consistency. if it’s too stiff, very carefully add a little more boiling water, drop by drop.
Spread the icing over the top of the cake, using a palette knife dipped in hot water. Will keep in an airtight container for up to 1 week.
Melt the chocolates in three different bowls in the microwave on High for 1 min or over a pan of simmering water. Spoon the melted chocolates into three plastic disposable piping bags. Snip the ends off and pipe 12 simple shapes on top of the cake. Leave to set. Just before serving, cut the cake into 12 squares and push in the candles.