• Preheat your oven to 350°F or 177°C, then grease two 9'' pans (or three 8'' pans). Line the bottoms with some parchment paper rounds and grease those, too.

Cake

• Dump in flour, sugar, baking powder, baking soda, espresso powder (optional), cocoa powder, and salt in a big bowl. Whisk them and set the bowl aside.
• Whip the eggs, oil, and vanilla at medium-high mixer speed. Pour in the buttermilk and give it another whirl.
• Mix wet ingredients into dry, add hot coffee or water, and give it a good beat or whisk on low. The batter will be pretty thin.
• Divide the batter between your pans. Bake them for 25-30 minutes or 'til a small toothpick poked in the middle comes out mostly clean with very few moist crumbs.
• Let the moist chocolate cake chill on a rack until completely cool.

Buttercream

• Beat butter at medium mixer speed for 1.5-2 minutes.
• Add the confectioners' sugar, heavy cream (3 tablespoons), cocoa powder, vanilla, and salt. Whip it at low speed (about 30 seconds), then cream it at high speed for 1 minute. No over-whipping.
• Add an extra ¼ cup of cocoa powder or confectioners' sugar (for too-thin frosting) and 1-2 tablespoons of cream (for too-thick frosting). Add more salt if necessary.

Assembling

• Level the dark chocolate cake with a big serrated knife.
• Place one of your cake layers on your serving plate, then slather on some delicious frosting. Do the same with the second chocolate cake layer. Sprinkle some mini chocolate chips (optional).
• Refrigerate for one hour before slicing. You can dig in right away or keep it chilling for 4 to 6 hours.