Melt the chocolate and butter together without boiling. Wait until the mixture becomes warm. If it's not warm, you won't be able to mix the rest of the ingredients.
Mix the chocolate and butter with the yolks mixture.
Whip the whites with an extra tablespoon of sugar until they hold shape
Slowly, fold the whites in the previous mixture.
Mix the flour, baking soda, and corn starch together and sift it by parts into the previous mixture, folding slowly.
Pour in a pan lined with butter and flour.
Bake for +-45 minutes at 180º C in a pre-heated oven.