Preheat oven to 400°F. Grease a large sheet pan (roughly 13×18 inches) or line with parchment paper.
Mix dry ingredients: In a large bowl, whisk together flour, sugar, baking soda, and salt. Set aside.
Whisk wet ingredients: In a separate bowl, combine ½ cup milk, vanilla, and beaten eggs.
Create the base: In a medium saucepan, melt 1 cup butter with 1 cup peanut butter over medium heat. Once it reaches a gentle boil, remove from heat.
Combine: Slowly stir the dry ingredients into the melted peanut butter mixture. Then add the milk/egg mixture and stir until smooth and well mixed.
Bake: Spread the batter evenly into the prepared pan. Bake for 20 minutes or until a toothpick inserted in the center comes out clean. Let cool completely in the pan.
Make the frosting: In a small saucepan, melt ½ cup butter with ½ cup peanut butter. Bring just to a boil, then remove from heat. Stir in 2 tablespoons milk, then gradually mix in the powdered sugar until smooth.
Frost the bars: Pour warm frosting over cooled bars and spread evenly. Allow to cool at room temperature until the frosting sets.
How to Store
Room temperature: Keep covered for up to 3 days.
Fridge: Lasts up to a week (let them come to room temp before serving).
Freezer: Wrap tightly and freeze for up to 2 months. Thaw overnight.
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