Place butter and sugar in a mixer and cream until smooth and pale. Slowly add the eggs, one at a time mixing well after each of them.
Meanwhile melt the chocolate in a bain marie or slowly in a microwave, and cool. When cool add the melted chocolate to the butter mixture and sift in flour and baking powder. Gently fold all in.
Now prepare the more delicate white chocolate and mascarpone layer. Place mascarpone cheese and add in a mixer and cream until smooth and creamy. Add the flour and mix again, finally add the white chocolate, still mixing. Pour about 2/3 of the chocolate brownie mixture in the prepared tin and then either pipe or just swirl the white chocolate mixture on top, pushing slightly into the brownie mixture as you go. Then spoon the remaining third of brownie mixture on top, mixing it and swirling it at the same time with the previous layer. Take a skewer or a fork and zig-zag the swirls to accentuate the wave effect.
Bake for 20 minutes. I like them slightly undercooked which ensures them to be moist and gooey.