Grease a 20cm cake tin with a little butter. Line the bottom with baking parchment.
Put the butter into a pan and place on a very low heat. When it starts to melt remove from the heat and beat lightly with a wooden spoon. You want to achieve a very soft almost runny consistency.
Put the butter and 190g caster sugar into a mixing bowl and beat together until creamy.
Break the eggs into a separate bowl. Add the vanilla extract and beat lightly with a fork.
Slowly beat the eggs into the creamed butter and sugar.
SIFT Sift together the flours and baking powder. Add to the mixture a third at the time, mixing thoroughly after each addition.
Pour the cake mixture into the tin and bake for 30 minutes until golden and coming away from the sides.
Turn the oven off and leave the cake in the oven for a further 5 minutes.
Meanwhile, rinse the blueberries under running water and place in a pan with 1 tablespoon of sugar and the juice of ½ lemon. Simmer for about 5-8 minutes until the berries start to burst. Stir in the mascarpone and set aside.
Turn the cake out and leave to cool on a wire rack or plate. Then slice it in half horizontally, spread the filling over one cake layer and top with the other layer. Sprinkle with icing sugar and serve.