Place butter, peanut butter, sugars, and buttermilk in a medium saucepan, and heat over medium heat, stirring frequently until sugar dissolved, and butter is completely melted.
While butter and sugar is heating, combine flour, baking soda, and salt together in a bowl and stir well.
When Peanut butter mixture is melted, remove from heat, and whisk in vanilla and eggs.
Stir in dry ingredients with a fork until well combined into a nice, smooth batter
Pour into a greased and floured 9×15 inch jelly roll pan
Place in center of preheated oven, and bake 20-25 minutes, until a toothpick inserted into the center comes out clean.
Remove from oven, and let rest.
While cake rests, combine Peanut butter, butter, and milk from the frosting ingredient list into saucepan, and heat to a boil.
Remove from heat and beat in vanilla and powdered sugar.
Pour over warm cake, and spread evenly with an offset knife.