Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in the vanilla and peanut butter.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Let cool completely before cutting into squares.
Notes
Store leftover brownies in an airtight container at room temperature for up to 3 days.