Preheat oven to 350 degrees, positioning a rack in the middle of the oven. Prepare a bundt pan by greasing (with butter, shortening, or cooking spray) thoroughly, making sure to get into all the cracks and crannies, and then dust with cocoa powder.

In a medium bowl, sift together flour and cocoa powder. Stir in baking soda, salt, and espresso powder.

In a separate bowl, using a hand-held or stand mixer, cream together the butter and sugar until very light and fluffy, about 3 minutes. Stir in vanilla, then add eggs and egg yolk, one at a time.

With the mixer on low speed, add half the flour/cocoa mixture, then half the buttermilk. Repeat. When all ingredients are combined, mix at medium speed for 15-20 seconds (no more) to whip just a bit of air into the batter.

Pour the batter into the prepared bundt pan, evening out the top with a large spoon or rubber spatula. Bake for 40-45 mins, until a cake tester comes out clean.

Cool briefly on a rack. When the pan is cool enough to handle, place a plate or cardboard cake round on the top and invert it (if needed, run a knife around the sides of the cake to ensure it will release). If not frosting immediately, cool just a bit more and then wrap in plastic so the cake doesn’t dry out.

Make the ganache:  place chopped chocolate in a medium bowl. In a small saucepan, bring cream just to a boil (watch carefully so it doesn’t boil over). Pour hot cream over chocolate, add vanilla, and stir until the ganache is uniformly dark and glossy. It will be fairly thin but will thicken as it cools. To speed this process, you can place it in the fridge for a few minutes. If you want a much thicker frosting-like ganache, whip it with a hand-held or stand mixer to your desired consistency.

Drizzle and/or spread ganache on top and sides of cake using a rubber spatula. Decorate with sprinkles. Store in an airtight container. Leftovers keep for several days.