Preheat the oven to 350˚F. Generously spray a 12x18-inch sheet pan with baking spray or rub with shortening then add a tablespoon of flour and tilt the pan in all directions to coat the shortening with flour.

Combine water, butter, and peanut butter in a medium-large, microwave-safe bowl. Cook on high power for 2-2½ minutes or until the butter is melted. Add the sugar and vanilla, then whisk until well combined. Set aside for 5 minutes to cool.

Add the eggs and whisk until smooth then add the baking soda and salt. Whisk until well combined.

Add the flour, whisking vigorously until lump-free then add the buttermilk and stir until well combined.

Pour the batter into the prepared pan and smooth to an even layer. Bake for 20-25 minutes, until a toothpick inserted in the center comes out clean and the top feels firm and springs back lightly when touched.

Cool for 20 minutes then top with the icing. Spread the icing with a knife to cover the surface. If desired, swirl the icing with the back of a small spoon for a pretty presentation. Serve warm or at room temperature.

Combine the butter, dark brown sugar, and peanut butter in a medium-large microwave-safe bowl. Cook on high power for 1 minute. Whisk well then return to the microwave for another minute.

Add 4 tablespoons of the buttermilk (or half and half or milk) and vanilla. Whisk well to combine. Add 3½ cups of powdered sugar and whisk until smooth. If too thick, add a bit more buttermilk. If too thin, add a little more powdered sugar and stir again until nice and smooth.