Sheet cake. It's a glorious thing. This here is a peanut butter version, and it doesn't really get any better than this. Oh wait...except when you smear melted chocolate on top of it.
For the cake: In a medium pot combine the peanut butter, water, and butter, and bring to a boil. Remove from heat.
In a separate bowl, mix together the sugar, brown sugar, flour, salt, and baking soda with a whisk. Add to the peanut butter mixture and combine.
Add the eggs, buttermilk, and vanilla to the bowl and mix well.
Pour into greased or parchment lined baking sheet that has an edge, such as a 12X17 jellyroll pan. Bake for 15 minutes at 375 degrees, or until a toothpick comes out clean.
For the Frosting: In a small pot bring the butter, buttermilk, and peanut butter to a boil. Remove pan from the heat and beat in the powdered sugar and vanilla. Pour frosting over the warm cake.
Notes
I didn't have fresh buttermilk, so I did the lemon juice trick. For the half cup called for in the cake, add half a tablespoon of lemon juice or vinegar to a liquid measuring cup, then fill it with milk up to the half cup mark. For the buttermilk in the frosting, I added one teaspoon lemon juice to a liquid measuring cup, then added 5 tablespoons plus 2 teaspoons of milk. Let sit 5 minutes before using.Source: The Girl Who Ate Everything