Preheat oven to 325.
In a 10” skillet, melt the butter and the chocolate over low heat.
In the bowl of a stand mixer fitted with the whip attachment, combine the yolks and sugar and beat on high until pale yellow, about 2 minutes. Reduce the speed to low and slowly mix in the flour. Once the flour is incorporated, slowly add the melted butter and chocolate.
Pour into a lightly greased 9” cake pan. Bake for 12-15 minutes.