Classic Chocolate Layer Cake

Steps

1. Preheat the oven to 175°C (350°F). Grease and line two 20 cm (8-inch) round cake pans with parchment paper and set aside.
2. Bring the eggs and buttermilk to room temperature, about 20–22°C. Brew the coffee and let it cool to 20–22°C before using.
3. In a large mixing bowl, whisk together the granulated sugar and light brown sugar until any lumps are broken up.
4. Sift the all-purpose flour, 60 g of the unsweetened cocoa powder, baking powder, baking soda, and about 3 g of the fine salt into the sugar. Whisk until the dry mixture looks even in color and finely blended.
5. In a separate bowl, whisk the eggs, buttermilk, neutral oil, 5 ml of the vanilla extract, and the cooled coffee until smooth and slightly frothy.
6. Pour the wet ingredients into the dry ingredients. Whisk gently just until the batter is smooth and slightly thin with no visible dry streaks; do not overmix to keep the crumb tender.
7. Divide the batter evenly between the prepared pans and smooth the tops so the layers bake level.
8. Bake at 175°C (350°F) for 25–30 minutes, until the centers spring back when lightly pressed, the edges are just starting to pull away from the sides, and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
9. Cool the cakes in their pans for 10 minutes, then turn out onto a wire rack, peel off the parchment, and cool completely at about 20–22°C until the layers feel set, moist, and light before frosting.
10. While the cakes cool, make the chocolate buttercream. Place the unsalted butter in a mixing bowl and let it sit until 20–22°C, then beat with a mixer for 2–3 minutes until very smooth and slightly fluffy.
11. Sift the powdered sugar, remaining 40 g unsweetened cocoa powder, and the remaining fine salt (a small pinch) into a separate bowl so the mixture is lump-free.
12. Add about half of the sugar–cocoa mixture to the butter and beat on low, then medium, until thick and smooth. Beat in the heavy cream and the remaining 5 ml vanilla extract, then add the rest of the sugar mixture and beat until the frosting is creamy, spreadable, and lightly fluffy. If too thick, beat in a few more milliliters of heavy cream; if too soft, beat in a little more powdered sugar.
13. To assemble, place one cooled cake layer on a serving plate. Spread about one third of the buttercream over the top in an even layer with a smooth, velvety surface.
14. Set the second layer on top, pressing gently so the layers adhere. Use the remaining buttercream to coat the top and sides of the cake, creating a smooth or swirled finish. When sliced, the cake should show even layers with a soft, moist crumb and rich chocolate frosting.