Mix all the ingredients for the dough in a food processor with the kneading blade until they form a soft dough. Wrap in cling film and rest in the fridge for about 30 minutes.
For the filling: using the food processor, beat the egg whites until stiff. Carefully mix in the ground nuts and add sugar (according to taste). Mix in some water until you get a soft consistency, but not too soft (shouldn't run off a spoon). Preheat the oven tp 180 C / Gas 4.
Roll the dough into a rectangle. Evenly spread the filling on top of it and roll the dough into a log. Form a ring and carefully place it on a baking tray covered with greased baking parchment. With a brush, brush the yolks evenly on top of the cake and, using a sharp knife, cut a zig-zag pattern.