These brownies are one of my favorite treats. By using a box mix you save a lot of time.
Family size dark chocolate brownie mix
ingredients on box to make brownies
1/4 cup peanut butter
2 heaping teaspoons of powdered sugar
Preheat oven and make the brownies as directed; pour in prepared 9x13 pan. In the measuring cup you used to measure the oil, spoon in 1/4 peanut butter and heat in microwave for about 15 seconds, just to melt it a little. (The left over oil in the glass measuring cup helps to prevent the peanut butter from sticking.)
Use a teaspoon (not the measuring kind, the coffee stirring kind) and add two spoonfuls of powdered sugar to peanut butter. Mix well.
Drop the mixture by spoonfuls evenly on top of brownie batter.
Use a knife and gently swirl/drag through the batter.