Preheat oven to 350 degrees F. Grease and line a 8"x8" baking pan, set aside.
In a medium mixing bowl whisk together the flour, cocoa, baking soda, baking powder, and salt, combine and set aside.
In a large mixing bowl whisk together the oil, sugar, eggs and vanilla extract until smooth. Gently whisk in the milk. Fold the dry ingredients into the wet, mix gently until smooth. Careful not to overmix. Transfer the batter into the prepared baking pan.
Bake in a preheated oven for 22-25 minutes or until a toothpick inserted comes out clean. Allow to cool completely before frosting.
For the Peanut Butter Buttercream:
In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl with a handheld electric mixer, beat the butter until light and fluffy, 4-5 minutes. Add in the peanut butter and mix another 1-2 minutes.
Add about half of the powdered sugar and beat on low until fully incorporated. Add in the remaining powdered sugar, vanilla extract, and salt. Beat on low speed again just until the powdered sugar is incorporated.
Scarpe down the sides of the bowl, add in the heavy cream and mix on low again until smooth. If the frosting is to thick, thin with extra cream, if it is to thin, add extra powdered sugar. Spread the buttercream over the cool chocolate cake. Top with crushed peanut if desired, slice, and enjoy!