Chocolate cake: Preheat oven to 350˚F. Lightly grease a 9”x 13” baking dish and set aside.
In a large mixing bowl whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Add eggs, buttermilk, water, oil, and extract (or paste) and whisk together until thick, even batter has formed.
Pour batter into prepared baking dish and bake for 35 to 40 minutes or until a toothpick comes out clean when inserted into the center of cake.
Allow cake to cool enough to handle, then transfer cake to a cooling rack and cool completely.
Frosting: Place butter and peanut butter into a mixing bowl and beat together with a hand mixer, until light and fluffy. Scrape down sides of bowl and add ½ cup sugar at a time until all sugar has been incorporated. Add vanilla, salt and heavy cream and continue to beat frosting until light and fluffy.
Assemble: Evenly spread frosting over sheet cake and top with shaved chocolate. Slice and serve.