01 -
Preheat oven to 350°F (177°C) and line a 9x9 baking pan with parchment paper.
02 -
Combine butter and 1/2 cup semi-sweet chocolate chips in a large microwave-safe bowl. Microwave for 30 seconds, stir well, then microwave for another 15 seconds and stir. Repeat in 15-second intervals until completely melted and combined.
03 -
Add cocoa powder to the melted mixture and stir well. Add granulated and brown sugar, stirring until fully incorporated.
04 -
Add eggs and egg yolk, one at a time, stirring thoroughly after each addition. Mix in the vanilla extract.
05 -
Sprinkle salt over the batter, then slowly mix in the all-purpose flour until fully combined. Fold in mini chocolate chips, if using.
06 -
Spread the batter evenly into the prepared pan. Bake at 350°F (177°C) for 30-35 minutes, or until a toothpick inserted in the center comes out slightly fudgy but not wet.
07 -
Allow the brownies to cool completely before frosting.
08 -
Using an electric mixer, beat together butter and peanut butter until smooth. Gradually add powdered sugar and mix until combined.
09 -
Stir in salt and vanilla extract. Gradually add heavy cream while mixing on low speed. Increase to high speed and beat until the frosting is smooth, scraping down the bowl if necessary.
10 -
Spread the peanut butter frosting evenly over the cooled brownies before serving.