Preheat your oven to 375°F. Grease an 11x17-inch jelly roll pan and set aside.

Make the Cake Batter. In a large saucepan, combine the butter, peanut butter, water and vegetable oil from the cake ingredients.

Bring the mixture to a gentle boil over medium-high heat, whisking until everything is completely melted and smooth.

Remove from the heat and allow the mixture to cool for a few minutes.

To the pan pan, while warm, whisk in the sugar, flour, eggs, vanilla, baking soda, and milk until the batter is smooth and no lumps remain.

Pour the batter evenly into the prepared pan.

Bake for 20 minutes, or until a toothpick inserted into the center comes out clean.

While the cake bakes, prepare the frosting. Wipe out the same saucepan—no extra dishes needed!

Combine the peanut butter, milk, and butter into the pan. Whisk over medium-high heat until melted and smooth, then bring to a gentle boil.

Reduce the heat to medium and slowly whisk in the sifted powdered sugar, one cup at a time, until silky smooth.

As soon as the cake comes out of the oven, pour the warm frosting evenly over the hot cake. Spread gently if needed.

Allow the cake to cool completely before slicing into squares. You will get 15-20 squares, depending on how large the slices are cut.