Preheat oven to 350°F (180°C). Spray an 8-inch square baking pan with cooking spray. Line pan with parchment paper, letting excess extend over sides of pan.

In a medium bowl, whisk together sugars and melted butter until smooth and well combined.

In another medium bowl, whisk together flour, cocoa, baking powder, salt, and espresso powder.

Slowly add eggs to sugar mixture, whisking until combined. Whisk in vanilla. Add flour mixture in three additions alternately with milk, beginning and ending with flour mixture, stirring just until combined after each addition. Fold in peanuts and chocolate chunks. Spread batter in prepared pan.

Bake until a wooden pick inserted in center comes out with a few moist crumbs, 35 to 40 minutes. Let cool in pan for 15 minutes. Using excess parchment as handles, remove from pan, and let cool completely on a wire rack.

Spread Peanut Butter Frosting on top of cooled brownies. Garnish with peanuts and chocolate chunks, if desired, and cut into bars. Refrigerate in an airtight container for up to 6 days. Let come to room temperature before serving.