Preheat the oven to 170°C (150°C Fan, Gas Mark 3). Grease and line an 18cm (7 inch) square cake tin. Break the Dark Chocolate into pieces and place in a heatproof bowl. Add the butter and stand the bowl over a pan of gently simmering water. Allow to melt, then remove from the water and stir in the sugar. Set aside to cool for 10 minutes.

Whisk in the eggs to make a thick and glossy mixture. Sift the flour and Baking Powder on top and add the White Chocolate Chunks, and carefully mix together.

Spoon into the prepared tin and smooth the top. Bake in the oven for 20 minutes until risen and lightly crusty on top – the mixture should be slightly soft underneath. Leave to cool completely in the tin (the cake will sink slightly) then remove and wrap in baking parchment and foil. For best results store for 24 hours to allow flavour and texture to develop.

To serve, unwrap and cut into 25 small pieces.