Preheat the oven to 180°C and grease and line a 30x23cm traybake tin.
Mix the cocoa and boiling water in a large bowl, then stir in the butter until smooth.
Add the remaining sponge ingredients to the bowl and mix until fully combined.
Pour the batter into the tin and bake for 30–35 minutes until the centre springs back when touched.
Cool in the tin for 10 minutes before transferring to a wire rack to cool completely.
Heat the cream in a saucepan, stir in the white chocolate until smooth, and leave to cool.
Pour the cooled icing evenly over the traybake.