Preheat your oven to 350°F. Prepare a 9 x 13-inch baking dish with nonstick cooking spray. Set it aside.

Whisk together 2 cups all-purpose flour, 2 ¼ cups granulated sugar, 1 teaspoon baking soda, and ½ teaspoon kosher salt in a large mixing bowl.

Combine 1 cup water, ½ cup (1 stick) unsalted butter, and ¾ cup creamy peanut butter in a medium saucepan over medium heat. Stir constantly until it comes to a gentle boil and everything is melted and smooth. Remove from heat.

Pour the warm peanut butter mixture over the dry ingredients and stir until mostly combined. Add ½ cup buttermilk, 2 large eggs, and 2 teaspoons pure vanilla extract, and mix until smooth. Don’t overmix.

Pour the batter into your prepared pan and smooth it out evenly. Bake for 25 to 35 minutes, or until the center is set and a toothpick comes out clean.

Once the cake is fully baked, remove it from the oven and let it cool completely in the pan on a wire cooling rack—this will take up to 1½ to 2 hours at room temperature. Don’t rush it! If the cake is even a little warm, the frosting will slide right off.

About 15 to 20 minutes before you’re ready to frost, prepare the buttercream. In a large bowl, beat 1 cup (2 sticks) softened unsalted butter and 1 cup creamy peanut butter together with a hand mixer or stand mixer until light and fluffy—about 2 to 3 minutes.

Add 2 teaspoons pure vanilla extract and a pinch of salt, then mix in 4 cups powdered sugar, one cup at a time. Add ¼ cup heavy cream gradually, beating until the frosting is thick, smooth, and spreadable. Add more heavy cream if needed to thin the frosting or add more powdered sugar if needed to thicken.

Once the cake is completely cool, use an offset spatula to generously spread the peanut butter frosting evenly across the top. If you like a thicker layer, pile it on—you’ve got plenty!