Preheat oven to 350°F. Grease (or line with non-stick foil) a 9"x13" brownie pan.
In a medium mixing bowl, whisk together the flour, cocoa, baking powder, and salt. Set aside.
2 cups flour, 1 cup cocoa, 1 tsp baking powder, 1 tsp salt
In a large mixing bowl, stir together the butter, sugar, and vanilla until well combined and the sugar begins to dissolve.
1 1/2 cup butter, 3 cups sugar, 1 tbsp vanilla
Add in the eggs and stir until well blended, the sugar should be completely dissolved the mixture will have a syrupy texture.
5 eggs
Add the flour mixture, mix just until combined. All dry ingredients should disappear into the mixture, but avoid over mixing. Reserve 2 cups of the batter (or about 1/3 the total batter), set aside for later use. Spread the remaining batter in the prepared pan.
Unwrap the peppermint patties and arrange of the surface of the batter with the edges just barely touching.
40 peppermint patties
Spread the reserved batter over the peppermint patties. Bake for 50 minutes or until the surface is glossy and set, the edges puffed and there may be some cracks, especially near the edges. The peppermint patties will be very melty and soft so most traditional doneness tests will not work.
Cool completely before cutting into squares (40 small or 24 large).
Notes
Mom's Note:Any type of soft filled chocolate can be used in place of the peppermint patties, such as peanut butter cups or Caramellos.My Note:My preferred brownie pan is a 9"x13" aluminized steel pan with sharp square corners. In that pan, I found that the 50 min bake time was too long by about 10-15 minutes. I recommend checking your brownies at about 35 minutes - you can always add more time, but there's no way to roll back the clock if you bake them too long. The original recipe calls for 24 small peppermint patties and tells you to space them about 1/2" apart. As I recall, mom did not leave any space between the candy when she made these. She would but the peppermint patties right up against each other (when I did that, it took about 40 of them instead of 24) or use whole Caramello candy bars in a solid layer. So, I adjusted the recipe ingredients and instructions to reflect how mom made these.