Whisk together the flour, sugar, baking soda, and salt and set aside.
Heat the butter and water in a medium/large saucepan until it comes to a boil. Take off the heat and whisk in the peanut butter and oil until smooth. Let cool for a few minutes, then whisk in the eggs, buttermilk, and vanilla until well blended.
Add the dry ingredients to the wet ingredients, and whisk until just combined.
Pour the batter into the prepared pan and bake for 40-45 minutes, until the top is golden and a toothpick inserted in the center comes out clean. Set the cake on a cooling rack while you make the frosting.
To make the frosting, put the butter, peanut butter, and buttermilk in a saucepan and bring to a full boil. Take off the heat and beat in the vanilla and powdered sugar, adding a cup at a time, until the frosting is smooth. I like to put it back on a gentle heat just to rewarm it, stirring constantly, before pouring over the cake. Note: if your frosting is very stiff and not pourable, add a little bit more buttermilk to thin it out.
Pour the hot frosting over the warm cake, working quickly because the frosting sets up immediately. Spread evenly over cake.
Garnish with peanuts.
Make it chunky! Yes, of course, go ahead and use chunky PB for this cake. I would still use smooth for the frosting, but that's your call.