In a saucepan heat milk until lukewarm and dissolve 1 tablespoon of sugar and the yeast.
In a large bowl mix the flour and the cubed 250g (8,8 ounces) butter by hand. The mixture will be quite crumbly.
Add the whole egg, 2 egg yolks, the dissolved sugar and yeast mixture, pinch of salt and the lemon zest. Knead thoroughly. Add more flour if necessary.
Divide the dough into 4 balls, cover and let rest for 30 minutes.
Heat the oven to 180°C (355°F). Butter and flour your baking tin. I used a 16×10 inch (40x25cm) pan.
On a floured surface roll out one part of the dough to the size of the baking tin, then lay it in.
Mix the ground walnuts (150g) with the powdered sugar (50g). Spread one third of the apricot jam on the first layer of dough. Sprinkle with third of the walnut-sugar mixture.
Roll out the second piece of dough. Put the jam on it and sprinkle with walnut-sugar mixture.
Roll out the third piece of dough. Spread the jam on it and sprinkle with walnut-sugar mixture.
Roll out the forth layer and place on top.
Bake the cake for 30 minute, until the top is light-brown. You can test it with a tootpick, if it comes out clean, then the cake is baked. Let it cool completely.
For the chocolate cover: in a smaller pan melt 100 grams (3,5 ounces or 8 tablespoons) butter, add 4 tablespoons of cocoa powder, 4 tablespoons of sugar and 4 tablespoons of water and stir until the sugar melts and the mixture thickens. Cover your cake with the chocolate sauce. The butter will make the chocolate have a glassy elegant look. Cut into diamonds and serve.