Add the egg yolks to the egg whites and combine with the melted chocolate and butter.

Delicately fold in the sifted flour.
Butter and flour 6 cups of a muffin pan. Pour batter three-quarters of the way up each cup.

Cover and refrigerate until well chilled, or freeze.

When ready to serve, bake in the oven at 400°F (200°C) for 5 to 6 minutes (8 to 10 minutes, if the cupcakes were frozen).

Serve immediately, so that the centers (or hearts) of the cupcakes are still soft.