Makes 12 servings
1. Have all ingredients at room temperature, 68 degrees F to 70 degrees F. Preheat the oven to 350 degrees F. Grease and flour two 9 x 2-inch round pans, or line the bottoms of the pans with waxed paper or parchment paper.
2. Whisk together 1 cup sugar, cocoa powder and 1/2 cup buttermilk or yogurt. Sift together flour, baking soda, and salt. Combine remaining 1/2 cup buttermilk or yogurt and vanilla.
3. In a large bowl, beat butter until creamy, about 30 seconds. Gradually add remaining 1 cup sugar and beat on high speed until lightened in color and texture, 3 to 5 minutes. Beat eggs in one at a time.
4. Beat the cocoa mixture into the butter mixture. Add the flour mixture in 3 parts, alternating with the buttermilk mixture in 2 parts, beating on low speed or stirring with a rubber spatula until smooth and scraping the sides of the bowl as necessary.
5. Scrape the batter into prepared pans and spread evenly. Bake until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Let cool in pans on a rack for 10 minutes. Slide a slim knife around the cakes to detach them from the pans. Invert cakes and peel off paper liners, if using. Let cool right side up on rack. Fill and frost cooled cake layers with Whipped Ganache Filling and Frosting, if desired.
1. Bring heavy cream to a boil in a medium-sized saucepan. Remove from heat and whisk in chocolate. Cover and let stand 10 minutes. With a rubber spatula, stir until perfectly smooth, scraping the bottom of the pan to be sure all the chocolate is melted. Cover and chill for at least several hours. (The ganache can be refrigerated for up to 5 days or frozen for up to 6 months). To use, beat on low to medium speed just until the ganache is thickened and begins to hold a shape; do not overbeat. Use immediately.