York Peppermint Pattie Brownies

This is a recipe that was sent to me by my good friend Michelle. She got the recipe from her wonderful Mother-in-law, Barbara Oldani. I haven't tried these yet, but I have been told they're absolutely delicious. Thanks, Barbara!
(I have added Celsius temperatures, for the Aussies.)

Yields 36

1½ cups butter or margarine, melted
3 cups sugar
1 tablespoon vanilla extract
5 eggs
2 cups all-purpose flour
1 cup Hershey’s cocoa
1 teaspoon baking powder
1 teaspoon salt
24 (1 ½ -inch) York Peppermint Patties (unwrapped)

(Australians could use Nestle Mint Patties, but you wouldn't need as many, because they're bigger. Maybe 10-12 would do.)

Grease a 9-by-13-inch baking pan. Preheat oven to 350 degrees (180 degrees Celsius), or 325 if using a glass baking pan (160-165 degrees Celsius). Beat together the butter, sugar and vanilla. Add eggs and beat until well-blended. Mix together the flour, cocoa, baking powder and salt. Stir it into the butter-sugar mixture and blend well.

Reserve 2 cups of the batter, spreading the remaining mixture in the prepared pan. Arrange the peppermint patties about ½ inch apart in a single layer over the batter. Spread reserved batter over the patties. Bake 50-55 minutes or until the mixture begins to pull away from the sides of the pan. Cool completely in pan and cut into 36 brownies.