Prepare the Oven and Pans: Preheat the oven to 350°F (175°C). Grease two 9-inch (23 cm) round cake pans. Line the bottoms with parchment paper, then lightly grease the parchment. This ensures that the cake layers release easily after baking.

Combine the Dry Ingredients: In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt. Break up any cocoa lumps and make sure the leavening is evenly distributed. This step is crucial for achieving a uniform rise in the cake.

Mix the Wet Ingredients: In a medium bowl, whisk the eggs, buttermilk, oil, and vanilla until smooth and evenly blended. Combining these ingredients thoroughly ensures that the batter will be consistent and well-flavored.

Begin the Batter: Pour the wet ingredients into the dry ingredients. Whisk gently until the flour is mostly incorporated. The batter will be thick at this stage, and a few small dry streaks are acceptable. This gradual mixing helps prevent overworking the flour, which can lead to a dense cake.

Add the Hot Coffee: Slowly pour in the hot coffee while whisking. Continue mixing until the batter is smooth and evenly chocolate-colored. The finished batter will be thin and pourable; this is expected. The hot coffee enhances the chocolate flavor and contributes to the cake's moist texture.

Divide the Batter: Pour the batter evenly between the prepared pans. Each pan should contain approximately the same amount so the layers bake at an even rate. Tap the pans gently on the counter to release large air bubbles, which helps prevent holes in the finished cake.

Bake the Cake Layers: Bake on the center rack for 28 to 32 minutes. The layers are done when the tops spring back lightly, the edges begin to pull away from the pans, and a toothpick inserted in the center comes out clean or with a few moist crumbs. This timing ensures that the cake layers are perfectly baked without being dry.

Cool in the Pans: Place the pans on wire racks and cool for 15 minutes. Run a thin knife around the edges if needed, then invert the layers onto the racks. Remove the parchment and turn the cakes right side up. Allowing the cakes to cool slightly in the pans helps them retain their shape.

Cool Completely: Let the cake layers cool for approximately 1 hour 30 minutes, or until they feel completely cool in the center. Frosting warm layers will melt the buttercream, so this step is essential for a clean finish.

Whip the Butter: Place the softened butter in a large mixing bowl. Beat on medium speed for 2 to 3 minutes, until pale, smooth, and creamy. Scrape down the bowl. Properly whipped butter is key to achieving a light and fluffy buttercream.

Add the Sugar and Cocoa: Add the powdered sugar and cocoa powder in three additions, mixing on low speed after each addition. The mixture will look dry and crumbly before the cream is added. This gradual incorporation helps prevent a sugar cloud and ensures a smooth frosting.

Finish the Buttercream: Add 1/3 cup (80 ml) of the heavy cream, the vanilla, and the salt. Beat on low until combined, then increase to medium-high and beat for 2 minutes. Add the remaining cream, 1 tablespoon (15 ml) at a time, only as needed. The buttercream should be smooth, fluffy, and easy to spread while still holding soft peaks.

Level the Layers if Needed: If the cake layers have noticeably domed tops, use a serrated knife to trim them level. Brush away loose crumbs before assembling. Leveling the layers ensures a stable cake and a professional appearance.

Fill the Cake: Place one cake layer on a cake plate. Spread approximately 1 cup of buttercream evenly over the top, stopping just short of the edge. This layer of frosting acts as a glue to hold the layers together.

Stack and Frost: Place the second cake layer on top, with its flattest side facing upward. Spread a thin layer of buttercream over the top and sides to trap loose crumbs. Refrigerate for 15 minutes if the cake feels unstable, then cover it with the remaining buttercream. This crumb coat helps create a smooth final layer.

Finish and Serve: Garnish with shaved semisweet chocolate, if using. Let the cake stand for about 15 minutes before slicing if it has been chilled. Cut into 12 slices with a sharp knife, wiping the blade between cuts to maintain clean edges. This final touch enhances presentation and makes serving easier.