Melt butter with crushed chocolate over water bath, stir in sugar, salt and vanilla sugar. gradually stir in eggs, then stir in the flour, nuts and white chocolate.
Line a pan (28 x 28 cm, approximately 11 x 11 inches) with baking paper. Pour the batter into the pan, smooth the surface and bake in preheated oven at 180°C on the middle rack about 35 minutes. Allow to cool in the pan.
Lift the brownies from the pan using the baking paper. To serve, cut into squares approximately 3.5 x 3.5 cm (approximately 1.5 x 1.5 inches).