Preheat oven to 350ºF. Line an 8 or 9-inch baking pan with foil or parchment paper (leave edges long to use as handles to lift finished brownies) and spray with nonstick spray.
In a medium saucepan, heat butter over medium-high heat until lightly browned, 3-4 minutes. Remove from heat and let cool about 5 minutes (transfer to a bowl if your pan retains heat.) Once cooled, whisk in eggs, egg yolk and vanilla extract until thoroughly combined. Whisk in brownie mix and baking powder until mixture is smooth and glossy. Fold in milk chocolate chunks. Spread brownie batter evenly into prepared pan
In a small bowl, mix peanut butter, melted butter and powdered sugar. Spoon dollops of peanut butter filling on top of brownie batter. Using the tip of a knife, gently swirl peanut butter filling into batter. Top with crushed pretzel pieces. Lightly press pretzels into batter with fingertips or the back of a spoon. Bake at 350° F for 25-30 minutes or until a toothpick inserted into the center comes out fairly clean. Cool completely, then refrigerate for 2 to 3 hours. Remove chilled brownies from refrigerator and lift from pan using foil or parchment paper handles. Discard foil or paper. Cut brownies into squares.
Brownies may be stored at room temperature, in an airtight container, for up to a week.
Makes one 8 or 9-inch pan, about 12-16 servings
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