When you combine chocolate and peanut butter, you can’t miss. These brownies are destined to be popular.
Adjust rack to lower third of oven and preheat oven to 350 degrees F. Line an 8x8-inch baking pan with aluminum foil. Press foil into corners and up the sides of the pan.
Whisk together the flour, baking powder, and baking soda in a small bowl. In a small, heavy-bottomed saucepan, melt the butter and the chocolate. Pour into a mixing bowl and stir in the brown sugar. Add the eggs, thoroughly blending into the mixture. Stir in the vanilla and peanut butter, then the flour mixture.
Spread batter evenly into prepared pan. Bake for 22 minutes. Cool in the pan on a rack. When cool, mark off 1-½-inch squares with the tip of a sharp paring knife. Using the handle of a wooden spoon, poke a hole into the center of each brownie; fill hole with peanut-butter chips, flat side up to fit flush with the surface of the brownies. Cut through the markings to form squares.