Cream the butter, golden caster sugar and light muscovado sugar together in a bowl until light and fluffy. Add the eggs one at a time, beating well between each addition. Sift the flour, cocoa powder and bicarbonate of soda into the mixture, then tip in any bran left in the sieve. Pour in the natural yogurt. Stir everything together until smooth.
Grease and line the base of a 28 x 18 cm (11 x 7 inch) tin. Spoon the mixture into the tin and bake in a preheated oven at 180°C (350°F), Gas Mark 4, for 20–25 minutes until just firm and shrinking away from the sides of the tin and a skewer inserted into the centre comes out clean. Cool in the tin for 5 minutes, then turn on to a wire rack to cool completely.
Sift the icing sugar and cocoa powder into a bowl, then pour in the melted butter and 2 tablespoons boiling water. Stir to a smooth spreading consistency. If it is too stiff, very carefully add a little more boiling water, drop by drop. Spread the icing over the cake top, using a palette knife dipped in hot water. Wrap a ribbon around the sides and tie in a bow. Finish with the chocolate caraque and the birthday candles.