Preheat oven to 350°F. Grease three 6-inch cake pans and line with parchment paper.
In a bowl of an electric mixer, stir together flour, sugar, cocoa powder, espresso powder, baking soda, baking powder, and salt.
Whisk together eggs, buttermilk, vegetable oil, and vanilla. Mix into dry ingredients until just combined. Scrape sides as needed
Heat water in the microwave for 45 sec - 1 minute until hot. Add slowly into cake mix while stirring, until just combined - be sure to not overmix! Divide batter evenly between the cake pans.
Bake for 24-28 minutes until the internal temperature reaches 195-200°F (or insert a toothpick that comes out dry). Let rest in pan for 5-10 minutes; remove and let cool on cooling rack completely.
Sour Cream Frosting: Beat room temperature butter with vanilla until smooth; add sour cream (bring sour cream out to come close to room temperature when you bake the cake) and mix to combine. Add powdered sugar cup-by-cup and mix to combine between additions. Beat until fluffy and smooth. Store in fridge until ready to frost cake (frosting is a little thinner than typical - letting it set up in the fridge helps!).
Stack the cakes up, frosting between each layer, around the outside, and on top. Decorate with drizzled chocolate or sprinkles as desired.
Notes
If you want to make a larger cake - make 1 in a 9-inch round pan (it will be thick and the center has sunk a little every time I've made it this way) OR divide between 2 9-inch round pans (thinner cakes, so be sure to reduce cooking time!)