Preheat oven to 375℉. Line the bottom of a half sheet pan (17.5”x13”) with parchment paper, spray parchment paper and sides of pan with non-stick spray.

In a small saucepan bring the peanut butter, water, and butter to a boil.

In a large bowl, combine the sugars, flour, salt and baking soda.

Pour the boiling mixture over the dry ingredients and mix well. Add in the eggs, buttermilk, and vanilla extract and mix until just smooth and combined. Pour evenly onto the prepared sheet pan. Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean. While the cake cools slightly, make the frosting.

For the frosting: In a saucepan bring the butter, peanut butter and milk to a boil, add in mini marshmallows and vanilla extract and beat it together until smooth, remove from heat. Sift the powdered sugar into a large mixing bowl, and pour boiling peanut butter mixture over the top, whisk vigorously until combined. This should be a pourable consistency, if it is too stiff add in another 1-2 tablespoons of milk if needed. Immediately pour over the warm sheet cake, spreading with a spatula to evenly cover. Top with chopped peanuts. Cool completely. Cut into 18-24 square servings.