Preheat your oven to 350°F and grease three 8-inch cake pans.

In a big bowl, mix together flour, cocoa powder, sugar, espresso powder, baking powder, baking soda, and salt.

In another bowl, mix eggs, milk, sour cream, vegetable oil, and vanilla extract.

Pour the wet ingredients into the dry ones and mix until it's smooth (around 15 seconds).

Divide this batter into the three pans.

Bake in the middle of the oven for 22-28 minutes, or until the cakes spring back when touched or a toothpick comes out clean.

Let the cakes cool on a rack, covering them loosely with a clean kitchen towel.

In a mixing bowl with a whisk, beat butter on medium speed for 1 minute until smooth.

On low speed, slowly add powdered sugar, cocoa powder, meringue powder, and salt, mixing well.

Add vanilla extract and increase speed to medium. Whip for 4-5 minutes until very light and fluffy, scraping the bowl occasionally.

Adjust the thickness with milk, if needed.

After the cake has cooled, remove it from the pans.

Put one cake layer on a cake board and frost with a layer of buttercream.

Repeat stacking and filling the cake layers, then frost all over with a thin "crumb coat" of buttercream. Refrigerate for 20 minutes.

Spread it all over with the last layer of butter.

Optionally, use extra buttercream to pipe swirls on top and decorate with chocolate squares.