Preheat oven to 350 F. Line a 9 x 13-inch baking pan with non-stick foil.
In a large bowl, whisk together flour, brown sugar, baking powder, baking soda, and salt until combined. Using low speed on an electric mixer, slowly blend in peanut butter and butter until mixture resembles coarse crumbs. Remove 1 cup of the crumbly mixture and set aside for use as the topping.
To the remaining mixture, add milk, eggs, and .
Blend on low speed, just until combined. Then increase mixer speed to medium and beat for 3 minutes. Pour into prepared pan and smooth batter to an equal depth. Sprinkle with and top with reserved peanut butter crumb mixture. Tap gently on the counter to set toppings.
Bake 40 to 45 minutes until a toothpick inserted in center of the comes out clean. Cool to room temperature before cutting.
Yield: 24 to 36 servings, depending on cut size.
Variations: Use all smooth peanut butter, all chunky or a mixture of both. You may fold an additional 1/2 cup of chocolate chips into the cake batter before topping for more chocolate punch.