Preheat oven to 350 °F (177 °C) and grease a 9x9 baking pan with cooking spray. Set aside.
In a saucepan, melt butter over medium heat then remove from heat.
Stir in sugar and vanilla extract. Add eggs, one at a time, and incorporating each egg in before adding another one.
In a large bowl, sift together cocoa powder, flour, baking powder, and salt.
Pour into saucepot and stir until all is combined and a thick batter is formed.
Pour half the batter into the prepared baking pan and place Reese's cups next to each other. There should be 4 in each row.
Pour the rest of the batter on top of the Reese's cups, covering them. Spread the batter gently with an offset spatula to make sure everything is covered.
Bake for 35-40 minutes then remove from oven and let brownies cool in pan until room temperature.
Using a paring knife, run it around the edge of the pan to loosen the brownies. Flip over onto cutting board and slice into squares.