1. Preheat oven to 350°F (175-180°C). Grease and line two to three 8 or 9-inch round cake pans.
2. Sift together dry ingredients: flour, cocoa powder, baking powder, baking soda, salt. Mix thoroughly.
3. In a separate large bowl, whisk eggs, sugar (granulated or brown), oil, milk (and sour cream if using), vanilla, and dissolved espresso powder or brewed espresso. Combine wet and dry ingredients, mixing gently until smooth. Expect a somewhat runny batter.
4. Divide batter evenly among prepared pans, tapping lightly to release air bubbles.
5. Bake for 25 to 40 minutes (depending on pan size and number of layers), until a toothpick inserted in the center comes out clean or with few moist crumbs.
6. Cool cakes completely on wire racks before frosting.
7. For espresso buttercream, whip softened butter on high speed until pale and creamy (about 5 minutes). Gradually add sweetened condensed milk or powdered sugar and meringue powder (if using), continuing to whip carefully to avoid splitting. Add vanilla, salt, and espresso dissolved in hot water or milk. Whip until buttercream is thick, glossy, and holds stiff peaks.
8. For ganache, heat cream just to a low boil, pour over chopped semi-sweet chocolate, let sit for a few minutes, then whisk until smooth. Cool slightly before pouring over cooled cake layers.
9. To assemble, level cake layers. For thinner layers and better cake-to-frosting balance, optionally cut each cake in half horizontally.
10. Alternate layers of cake and frosting or ganache. Coat the outside with remaining frosting. Chill if desired.
11. Decorate with chocolate curls, shavings, or espresso beans as preferred.