Melt the chocolate and butter together over hot water. Cool slightly. Gradually add the sugar to the eggs, beating thoroughly after each addition. Stir in the flour, then add the vanilla and nuts. Spread in a greased 9-inch square pan and bake in a slow oven (325 degrees) about 40 minutes. Cool in the pan. Cut into squares.
Bake brownies as above, then arrange 10-20 chocolate peppermint patties over the hot brownies; return to the oven for about 3 minutes to soften. Then spread to cover the entire top of the brownies. Cool and cut.