Preheat oven to 350°F (175°C). Grease two 8-inch round cake pans and line bottoms with parchment; grease parchment and lightly dust with flour, tapping out excess.

In a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Whisk in granulated sugar until evenly combined.

In a separate medium bowl, whisk together the hot brewed coffee (hot but not boiling), buttermilk, vegetable oil, eggs, and vanilla extract until smooth. The coffee will bloom the cocoa and thin the batter slightly.

Make a well in the dry ingredients and pour in the wet ingredients. Stir gently with a spatula or whisk until just combined and no large streaks of flour remain. Do not overmix.

Divide batter evenly between prepared pans (about 2 cups batter per pan). Smooth tops with an offset spatula.

Bake in the preheated oven for 25–30 minutes, rotating pans halfway through, until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).

Allow cakes to cool in pans on a wire rack for 10 minutes. Run a knife around edges, invert onto racks, remove parchment, then right-side up to cool completely before frosting (about 1–1.5 hours).

Mocha Buttercream Frosting

Dissolve the instant espresso powder in the hot water; set aside to cool slightly. This makes a concentrated espresso slurry to flavor the frosting.

In the bowl of a stand mixer fitted with the paddle or using a handheld mixer, beat the softened unsalted butter on medium speed for 2–3 minutes until pale and creamy.

With the mixer on low, add the powdered sugar a cup at a time, mixing until incorporated between additions. Scrape down the bowl as needed.

Add the cocoa powder, dissolved espresso, heavy cream, salt, and vanilla extract. Increase speed to medium-high and beat for 2–3 minutes until light, fluffy, and spreadable. If frosting seems too stiff, add up to 1 tablespoon more cream; if too soft, add 2 tablespoons more powdered sugar.

Taste and adjust: add a pinch more salt if overly sweet or 1/2 teaspoon more espresso dissolved in 1 teaspoon hot water for stronger coffee flavor.

Assembly & Garnish

If cake layers have domes, level them with a serrated knife or cake leveler to create even layers.

Place the first cake layer on a serving plate or turntable. Spread a generous layer (about 3/4 to 1 cup) of mocha buttercream over the top, smoothing to the edges.

Top with the second cake layer and press gently to adhere. Apply a thin crumb coat of frosting over the top and sides (about 1/2 cup). Chill the cake in the refrigerator for 15–20 minutes to set the crumb coat.

Remove the cake from the fridge and apply the remaining frosting, smoothing the sides and creating decorative swirls on top with an offset spatula or the back of a spoon.

If using, sprinkle chocolate shavings and/or chopped toasted nuts around the top edge or center of the cake. Optionally, dust lightly with cocoa powder for extra presentation.

Allow assembled cake to sit at room temperature for 20–30 minutes before slicing for cleaner cuts, or refrigerate for up to 3 days (bring to room temp before serving).