Preheat oven to 350°F and line an 8x8-inch pan. Mix butter and sugars until smooth. Add eggs and vanilla. Combine dry ingredients separately, then stir into wet mixture. Fold in chocolate chips. Spread half batter into pan, add peanut butter cups, then cover with remaining batter. Bake 35–40 minutes. Cool before slicing.
Notes
Do not overbake. Brownies stay fudgy when slightly underdone in the center.