Preheat oven to 190 degrees C and butter large muffin tins.
Beat the egg yolks, eggs, sugar and flour until smooth, using an electric beater.
Melt the butter and chocolate together in a double boiler, stirring until smooth. Beat into the batter for 5 minutes.
Pour batter into the muffin tins and bake 8-10 minutes. The centre of the cakes will be very liquidy and thus hot, so be careful. Let sit 2 minutes then carefully unmold onto plates. You can also serve them cold and the centres will be creamy.