Separate eggs. Mix the egg yolks with the sugar until frothy. Add the flour, keep stirring and mix with the melted butter. Melt the chocolate in a saucepan with the espresso. Add to the dough. Beat the egg whites until stiff and mix in. Grease a round cake tin and sprinkle with flour. Bake in the preheated oven for about 40 minutes at 170 ° C. Immediately remove the cake from the pan and serve with vanilla sauce.