Preheat oven to 350°F. Grease & flour three 8" round cake pans (I use homemade cake release) & line the bottoms with parchment paper.
In the bowl of a stand mixer, sift all dry ingredients (flour, sugar, cocoa powder, baking soda, baking powder, and salt). Whisk to combine.
In a medium bowl, whisk together the eggs, buttermilk, coffee, oil and vanilla.
Add the wet ingredients to the dry and mix for 2-3 minutes on medium. Scrape the sides of the bowl as needed. Note: the batter will be very thin.
Divide the batter evenly amongst the 3 cake pans (I used a kitchen scale to ensure evenness).
Bake for 20 minutes and rotate the pans in the oven. Continue to bake until a toothpick inserted into the center comes out mostly clean (a few crumbs) about 10-15 more minutes.
Cool the cakes (in the pans) on wire racks for 20 minutes, then carefully turn them out onto cooling racks to cool completely.
FROSTING
Sift the powdered sugar. Place the peanut butter, butter, vanilla and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix on medium-low speed until creamy. Turn speed down to low and add in the powdered sugar scraping down the bowl with a rubber spatula as needed. Add in the heavy cream and beat on high until the mixture is light and smooth. *
Notes
*I whipped this for quite a while as I wanted a lighter frosting.Adapted from Brown Eyed Baker.