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.

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.

In a medium bowl, whisk the eggs, buttermilk, oil, and vanilla until smooth and evenly blended.

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.

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.

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.

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.

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.

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.

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.

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.

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.

If the cake layers have noticeably domed tops, use a serrated knife to trim them level. Brush away loose crumbs before assembling.

Place one cake layer on a cake plate: Spread approximately 1 cup of buttercream evenly over the top, stopping just short of the edge.

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.

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.