Bain-marie melt butter and chocolate.
Pour flour, sugar, baking powder, and one pinch of salt in a pan and amalgamate. Add chocolate and eggs and stir with a spoon until obtaining a uniform mix. Pour the mix in a 24cm squared or rectangular tray.
Pour approximately 9 spoonful of jams over the chocolate mix, in different places. With a meat skewer spread the two mixes in order to obtain the marble effect.
Cook in pre-heated oven at 170°C for approximately 35 minutes. Make sure to preserve the mixture from totally drying. Completely cool down before cutting in squares and serving.