Make the Cake

1. Preheat your oven to 350°F. Line a 15x10x1 (jelly roll-style) pan with parchment paper. Coat with nonstick spray and set aside.
2. In a large bowl whisk together the flour, sugar, baking soda, and salt. Set aside. 
3. In a small bowl whisk together the eggs, vanilla, and buttermilk. Set aside.
4. Place water and butter in a small saucepan over medium heat. Bring to a boil and remove from the heat. Stir in the peanut butter and oil, mixing until combined. Add this into the flour mixture and whisk until smooth. Then whisk in the buttermilk mixture until evenly blended. 
5. Pour the batter into the prepared pan and bake for 20 – 25 minutes until the center is set and a toothpick inserted comes out clean. 

Make the Icing

1. Stir together the evaporated milk, granulated sugar, salt, and butter in a medium sauce. Bring to a boil over medium heat, and boil for 2 minutes stirring constantly. Remove from the heat and stir in the peanut butter, marshmallows, and vanilla until combined and the marshmallows are melted. 
2. Pour the icing onto the warm cake, spreading out evenly using an offset spatula. 
3. Allow the cake to cool completely before slicing and serving.