Preheat the oven to 140C/280F/Gas 1. Grease and line a 23cm/9in square cake tin.
For the cakes, in a large bowl, whisk the butter and sugar together, using an electric whisk, until pale and fluffy.
Gradually add the eggs, whisking continuously, waiting until each addition has been fully incorporated into the mixture before adding the next.
Carefully fold in the flour and the baking powder until well combined.
Spoon the cake batter into the prepared cake tin. Transfer to the oven and bake for 25-30 minutes, or until the sponge has risen and a skewer inserted into the centre of the cake comes out clean. Set aside to cool, then cut into ten 2.5cm/1in squares.
For the icing, bring a little water to the boil in a pan, then reduce the heat until the water is simmering. Suspend a heatproof bowl over the water (do not allow the base of the bowl to touch the water). Add the butter, dark chocolate and icing sugar and stir until glossy and smooth.
Gradually add the boiling water until the mixture has loosened. (You may not need to use all of the water. The icing mixture should be runny enough to drip down the sides of each cake square.)
Dot each cake square with a teaspoon of mascarpone. Spoon over the chocolate and butter icing to cover each cake square. Set aside for 20-30 minutes, or until the icing has set.
Spoon the melted white chocolate into a piping bag and drizzle decorative lines all over the iced fondant fancies. Set aside to cool.
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