Preheat oven to 350°F (177°C). Grease 11×7-inch pan and set aside.

In a saucepan over medium heat, melt the butter and brown sugar, stirring until smooth. Remove from heat, and stir in the peanut butter. Set mixture aside to cool.

In a large bowl, whisk together flour, baking powder, baking soda, and salt.

Add eggs, one at a time, to the peanut butter mixture, mixing well after each egg. Stir in pure vanilla extract. Add this mixture to the prepared dry ingredients, and stir until batter is smooth. Fold in 1/2 cup of Reese’s Pieces, leaving 1/4 cup to sprinkle on top.

Pour batter into the greased pan, spreading batter evenly. Sprinkle with remaining 1/4 cup Reese’s Pieces.

Bake for 25-30 minutes or until top is golden brown and edges are lightly browned. Allow 20 minutes to cool and cut into squares. Cover and store leftover blondies at room temperature or in the refrigerator for up to 1 week.