Preheat the oven to 180C/160F/Gas 4 Grease and line the traybake tin with non-stick baking paper.

To make the traybake, break 100g of plain chocolate in a small bowl; sit the bowl over a pan of simmering water and gently stir until melted and smooth. Leave to cool slightly.

Sieve the cocoa powder into a stand mixer or large mixing bowl. Add the boiling water and mix, to a smooth paste. Add the eggs, baking spread, caster sugar, flour, baking powder and milk. Whisk together using a stand mixer or hand mixer until light and fluffy. Mix in the melted dark chocolate and white chocolate chips.  Spoon the mixture into the prepared tin and level the surface. 

Bake in the preheated oven for about 30 minutes until well risen and coming away from the sides of the tin. Leave to cool on a wire rack.

Whilst the cake is cooling, make the icing. Break the plain chocolate into pieces and add to a heatproof bowl, pour over the cream. Melt over a pan of simmering water, stirring until runny and smooth. Leave the bowl in a cool place to thicken to a thick spreading consistency.

Spread the icing over the top of the cake. Sprinkle with the white chocolate chips. 

Serve in squares dusted with icing sugar (optional)