Preheat oven to 350°F. Spray three 4” round by 2” high cake pans or two 6" round 2" high cake pans with non-stick cooking spray and line the inside bottom of each pan with parchment paper. Set these cake pans aside for now.
In a medium bowl use a whisk to sift together flour, sugar, cocoa powder, baking soda, baking powder and salt until well combined.
In a separate medium bowl use an electric mixer to whip together buttermilk, vegetable oil, egg and vanilla extract. Beat on medium speed until combined.(about 30 seconds).
Lower the speed of the mixer and gradually add the sifted dry ingredients into the wet ingredients. Beat on low speed just until all of the ingredients are combined and no more lumps remain. This takes less than a minute of beating on low speed.
Dissolve the espresso into the hot water. Use a large rubber spatula to fold into the cake batter until fully combined. Do not use the beaters for this step. The cake batter will be runny and that’s exactly what we’re looking for here. (Hot Water: This helps bloom the chocolate flavor and dissolves the espresso powder, the sugar and cocoa solids more effectively resulting in a smooth velvety texture.)
Divide the batter evenly between the prepared cake pans. It’s important to measure accurately so that the cakes don’t overflow out of the pans during baking.
Bake in preheated oven for 20 to 22 minutes or until a toothpick inserted into middle of the cakes comes out clean.
Remove cakes from oven and let cakes cool in pans for 15 minutes then remove from pans and transfer to cooling racks to cool completely.
Place the cooled cakes in the freezer for 15 minutes before frosting. This step is important because this cake has a tender texture and they need to be firm before frosting to prevent them from crumbling.
Pro Tip: If the cakes are domed, after removing cakes from the freezer, use a large serrated knife to gently cut the dome tops off cakes so they all have a flat top.
Chocolate Buttercream Frosting Directions: In a large bowl use an electric mixer to cream butter until smooth. To the bowl add in the powdered sugar, cocoa powder, heavy cream, vanilla extract and salt. Beat on high speed until light and fluffy. Add additional cream 1 Tbsp at a time to thin to spreading consistency is reached.
Place a thin layer of frosting on the bottom of one cake layer then place this frosted side down on the cake stand or cake board to secure the cake.
Add a generous layer of frosting on top of that first layer then place the second layer on top. Repeat this process with all three layers. (Pro Tip: Once the layers are assembled, you can apply a crumb coat to the cake. This is a thinly applied amount of frosting to the top and sides, then pop the cake into the freezer for 15 minutes to set before applying the final layer of frosting.)
Frost top and sides of cake, and decorate the top as desired with swirls of buttercream, sprinkles, cookie crumbs or nuts.
Optional Piping As Pictured: Using an icing bag and a Wilton 1M open star tip to add frosting swirls to the top edge of the cake. Decorate with sprinkles as desired.