Melt two squares of chocolate, add three tablespoonfuls of butter, one cupful of powdered sugar and one-half cupful of milk. Stir and cook until it begins to thicken. Add the yolks of two eggs, well beaten, in one-half cupful of milk. Continue beating until it is almost jellylike. Set aside to cool. When almost cold add one teaspoonful of baking soda dissolved in a little water. Stir in one and two-thirds cupfuls of flour, well sifted. Bake in two square layers. It will take only about seventeen minutes to bake.