Preheat oven to 350-degrees F and line a 9-inch square baking pan with parchment paper (let a little hang off the sides so you can pull them out easily)
Melt butter in a small saucepan, remove from heat and add peanut butter, and milk. Stir continuously until smooth and then let cool slightly.
In two bowls: Whisk together flour, baking powder and salt, set aside. In a separate bowl mix together eggs, brown sugar, and vanilla extract by hand. Gradually whisk in melted butter/peanut butter mixture until combined and smooth.
Add flour mixture into peanut butter mixture, stir unti just combined. Do not overmix.
Spread batter evenly into prepared 9-inch square baking pan. Arrange mini peanut cups 4x4 or 3x3 depending on how big you want your squares.
Bake at 350-degrees for 20 minutes, or until cake tester or wooden toothpick inserted in center comes out with moist crumbs.
Let pan cool before cutting or throw in the freezing if you're in atimecrunch. Cut into even squares and enjoy!