Preheat the oven to 180C/160C Fan/Gas 5. Grease a 30x23cm/12x9in traybake tin and line with baking parchment.
Put the cocoa and boiling water in large bowl and mix. Add the butter and mix until smooth. Add the remaining sponge ingredients and mix until fully incorporated.
Pour into the tin
and bake for 30-35 minutes
or until the traybake springs back when pressed lightly with a finger in the centre and is beginning to shrink away from the sides of the tin.
Leave to cool in the tin for 10 minutes
then turn out
carefully remove the parchment and place on a wire rack to cool completely.
To make the icing
gently heat the cream in a small heavy bottomed saucepan. Add the white chocolate and stir until smooth. Leave to cool.
Pour the icing over the traybake in an even layer.
