1. Gently melt the butter, remove from heat and stir in the dark and milk chocolate.
2. Stir egg yolks, powdered sugar, and salt into the chocolate mixture. Transfer the chocolate mixture to a piping bag.
3. Line a bread loaf pan with baking paper.
4. Place a little chocolate mixture at the bottom of the pan and add biscuits. Add chocolate mixture on top and more biscuits. Continue until there are 4 layers of biscuits, finishing with chocolate mixture on top (optional: sprinkle with chopped, toasted hazelnuts). Wrap the cake well in plastic wrap and refrigerate.
