1. Preheat the oven to 180°C (350°F) and grease a ring-shaped mold.
2. In a bowl, mix the egg yolks with the sugar until the mixture becomes pale yellow.
3. Meanwhile, melt the chocolate in a double boiler with the coffee.
4. When the chocolate is soft, add the butter in small pieces, let it melt, then mix with a fork to obtain a smooth dough.
5. Add this mixture to the egg preparation, then incorporate the sifted flour.
6. Beat the egg whites until stiff peaks form and gently fold them into the preparation.
7. Pour the batter into the mold, filling it two-thirds of the way. Slide into the oven and bake for 25 to 30 minutes.
8. Monitor the cooking process, covering the cake with aluminum foil if the top tends to brown too much.
9. Unmold onto a platter and let the cake cool completely.
10. Fill the center with small eggs and colored sugar decorations.
11. Serve as is, or accompany with coffee or vanilla flavored custard.