Crumble the dark chocolate into smaller pieces. Melt in a bain-marie. Add the butter, sugar and eggs, lightly beaten. Gradually add the flour while stirring to prevent mixture from lumping. Pour the filling into the pastry nests and sprinkle the surface with grated walnuts. Bake in a preheated oven at 200°C for 20 minutes, on the lowest shelf of the oven. Sprinkle with powdered sugar and serve.

For making Filo Pastry Nests:
Cut the pastry sheets lengthwise into 2 strips, and cut each strip in 3 pieces. Butter a muffin tin or 12 mini individual tart forms. Brush 6 sheets with butter and place on top of each other to form 6 layers. Place them in a muffin case, and softly press down with your hand to form a nest inside the case. Repeat the process with all pastry sheets until you make a nest in each case.