Step 2

Preheat oven to 350°F (180°C). Butter three 9-inch cake pans and line with parchment paper.

Step 3

In a medium bowl, sift flour with cocoa powder, baking soda, baking powder and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat sugars with butter on medium-high until light and fluffy, 1 minute. Reduce speed to medium-low and stream in melted chocolate until combined. Add eggs, one at a time, just until combined. Beat in vanilla. Add flour mixture into butter mixture in 3 parts, alternating with buttermilk in 2 parts. Divide batter between pans, smoothing tops.

Step 4

Bake in center of oven until a tester inserted into the center of cakes comes out clean, 35 to 40 minutes. Transfer to a rack. After 10 minutes, run a knife around sides of cakes. Turn out of pans. Invert and cool on racks, about 1 hour. Cover with plastic wrap and chill in refrigerator, 2 more hours.

Step 5

To assemble a layered cake, trim tops of cakes if needed. Place 1 cake round on a cake stand. Spread with ½ cup of the buttercream (see instructions, below). Lay another cake round on top. Repeat spreading and layering. Frost sides and top of cake.

Step 6

Chocolate Buttercream Frosting – In the bowl of a stand mixer fitted with the paddle attachment, beat butter with icing sugar on medium until combined, 1 minute. Reduce speed to medium-low and stream in melted chocolate until combined. Increase speed to medium-high and beat until very light and fluffy, 5 minutes.

Step 7

Beat in vanilla.