Melt butter in a heavy saucepan. Add in sugar, brown sugar, molasses and cream. Bring to a boil, and continue to boil for 2 1/2 minutes, stirring constantly. Add the chocolate, and boil on moderate heat for another 5 minutes. Remove from heat, and add in the vanilla. Stir until the mixture thickens, and transfer into a buttered mold. Cut when cool.