Pre-heat the oven to 180°C/350°F/Gas Mark 4. Grease and line two 2 x 9 inch (23cm) cake tins with a removable base with parchment paper. Sieve the flour, cocoa and bicarbonate of soda into the bowl of a stand-alone mixer. Add the sugar and mix well.
Measure the wet ingredients in a measuring jug. Make a well in the centre of the dry ingredients and pour in the contents of the jug. Beat together in the mixer until you are left with a smooth mixture. Spoon the cake batter into the lined tins and bake for 25-30 minutes, or until risen and firm to the touch. Remove from the oven and leave to cool before transferring to a wire rack until completely cool.
To make the frosting, melt the chocolate in a bowl placed on top of a saucepan filled with a little water boiling over a low heat. Remove from the heat to cool for about 10 minutes. Place the butter and cream cheese in a bowl and beat until fluffy and combined.
Mix through the melted white chocolate and then gradually sift and beat in the icing sugar until you have a smooth, spreadable mixture.
Slice the the cake layers in half horizontally and then place one on a cake stand and spread over a little of the frosting. Repeat this with all the layers of the cake until it is assembled. If you want, you can spread the frosting on the outside of the cake too, this will use the full quantity of frosting.