Preheat oven to 350°F. Whip egg whites with apinch of salt till they are foamy and stand in peaks.
Add the egg yolks, one by one, beating after each addition for about 2 minutes.
Add the granulated sugar and whip for another 2 minutes. Add flour, little by little, and stir until the batter is very smooth.
Butter a special indiáner form (or if not available, a muffin tin). Fill 12 forms to three-fourths capacity with the batter. Bake in the preheated oven for 10 to 12 minutes.
When the cakes are cool, cut them across into halves, and scrape out the center of each half.
Make the glaze: Place the chocolate in the top part of a double boiler and soften over hot water, while stirring. When chocolate is completely melted, remove from heat. When cool, whip in the butter.
Dip the top halves of the cakes into the chocolate glaze, and place them in refrigerator to harden.
Whip the heavy cream till it becomes stiff, then mix in the vanilla sugar.
Fill the hollowed-out unglazed halves of the cakes with the sweetened whipped cream. Place chocolate-glazed halves on top. Whipped cream should show through around the middle, resembling grinning teeth.