In a standing food mixer fitted with a whisk attachment, whisk together the egg yolk, egg white and 45g of the sugar until foamy. In a separate bowl, mix together the flour, baking powder, cocoa and remaining 45g of sugar
Melt the chocolate and butter in a microwave and stir into the flour mixture. Stir in the water, followed by the whisked eggs and sugar
Pour into the prepared baking tray and bake in the oven for 15 minutes. Remove from the oven and allow to cool in the tray