In a large mixing bowl, combine 130g of flour, 250g of sugar, and 100g of ground walnuts. While stirring continuously with a wooden spoon, gradually add 125g of melted butter to the mixture. Continue mixing and fold in 5 egg whites that have been whipped to stiff peaks. Once combined, transfer the batter to a greased cake pan. Bake in a preheated oven for 30 minutes. Once the cake is done, spread any jam of your choice on top, and then use a pastry bag to create a lattice pattern with the remaining whipped egg whites. Return the cake to the warm oven for a few more minutes.