Melt the chocolate and butter in a double boiler (or microwave). Cool to room temperature.

Beat the eggs, gradually add the sugar, then vanilla, and finally add slowly the melted chocolate to the beaten eggs, until well blended.

In a large bowl, mix the flour, baking powder, salt and walnuts. Add the eggs/chocolate mixture, and mix until blended.

In another bowl, mix well the mascarpone cheese and butter. Add sugar, egg, flour and vanilla. Mix until blended.

Preheat oven to 360 F (180 C).

Layer a square 9" baking pan with parchment paper, and butter the paper and the sides of the pan. Pour 2/3 of the chocolate mixture, and flatten with a spatula. Add the vanilla mixture, flatten. Using a spoon, add the remaining 1/3 of the chocolate mixture. Using a knife, mix slightly the two top layers, to create a marble effect. Don't overmix.

Bake 30 to 35 minutes. Cool before cutting to squares.