For the wontons
For the chocolate butter cream
Preheat the oven to 180°C/350°F/Gas mark 4. Lightly grease a 20cm/8in square cake tin, and line with greaseproof paper.
Cream the butter and sugars in a small bowl with an electric beater until light and fluffy. Add the eggs gradually, beating thoroughly after each addition. Beat in the vanilla and jam. Transfer to a large bowl.
Sift together the flour, cocoa and bicarbonate of soda. Using a wooden spoon, fold the flour mixture into the butter alternately with the milk. Stir until the mixture is just combined and almost smooth.
Pour into the prepared tin, and smooth the surface. Bake for 45 minutes until a skewer inserted into the centre of the cake comes out clean. Remove from the oven and leave in the tin for 15 minutes before turning onto a wire rack to cool completely.
To make the butter cream, stir the ingredients in a small pan over a low heat until smooth and glossy. Spread over the top of the cooled cake using a flat-bladed knife. Allow to set.