Peanut Butter Sheet Cake

Preheat oven to 350 F and grease a cookie sheet pan (13×18''). Set aside.
Add butter and water to a small saucepan over medium heat. Bring mixture to a boil, remove from heat and mix in peanut butter until completely melted in.
In a large bowl whisk together the sugar, brown sugar, flour, baking soda and salt. Pour melted butter/peanut butter over the dry ingredients while they are still warm. Whisk together until all combined.
In a measuring cup or small bowl whisk the eggs, buttermilk and vanilla. Mix into the cake batter.
Pour batter into prepared pan. Bake for 17-22 minutes, or until edges are pulling away. When there is about 5 minutes left before the cake is done baking, start the peanut butter frosting.
Add butter, peanut butter and milk to the same small saucepan and bring to a boil over medium heat. Remove from heat and pour into a medium mixing bowl with vanilla, salt and powdered sugar. I like to mix with electric beaters on low to make sure it's smooth. You do not want to start the frosting too soon or it will start to thicken and harden and you want that to happen on top of the cake.
Pour frosting over cake after it's cooled for 3-5 minutes, and use a spatula to spread evenly over the cake. The cake needs to be warm enough that the frosting can spread easily over the cake. Top with chopped peanuts and flaked salt.