Stir together butter, sugar and vanilla in a large bowl. Add eggs. Beat well. Stir in flour, cocoa, baking powder and salt. Blend well. Reserve two cups of batter. Spread the remaining batter in a greased 9 x 13 inch pan.

Arrange peppermint patties evenly on top of the batter. Spread reserved batter over the top. Bake at 350 degrees (for a metal pan) or 325 degrees (for a glass pan) for about 40 minutes or until set and a sharp knife comes out relatively clean.

Cool completely before cutting into large squares. These freeze beautifully.