Preheat the oven and prepare the cake tin.

Melt the 60 g of dark chocolate and butter together in a bowl over hot water.

Stir in the sugar, hot water, plain flour, self-raising flour, and cocoa until combined.

Add the egg and mix until smooth.

Pour the mixture into the prepared cake tin and bake until set.

While the cake is baking, prepare the mousse by melting the remaining dark chocolates.

Whip the thickened cream until soft peaks form, then fold it into the melted chocolate.

Allow the cake to cool, then layer the mousse on top.

Refrigerate until set.