Preheat the oven to 350 F, and spray the bottoms of three 8-inch cake pans with non-stick spray. If making half the recipe, use three 6-inch cake pans.

In a large bowl, sift together the flour, sugar, cocoa powder, espresso powder, baking soda, baking powder and salt.

Separately, whisk together the eggs, milk, sour cream, oil and vanilla. Add the wet ingredients to the dry, and whisk until combined, about 15 seconds.

Divide the batter between the pans. Bake on the center oven rack for about 22-28 minutes, until the tops of the cakes spring back when lightly touched, or a toothpick inserted in the center comes out clean.

Set the pans on a cooling rack, cover loosely with a clean kitchen towel, and cool completely before frosting.

In the bowl of your stand mixer fitted with the whisk attachment, beat the butter on medium speed for 1 minute until smooth.

With the mixer on low, gradually add the powdered sugar, cocoa powder, meringue powder and salt, mixing to combine.

Add the vanilla, and increase the speed to medium (#4-6 on a Kitchen Aid mixer); whip for 4-5 minutes until very light and fluffy, scraping the bowl down occasionally, and only adding the milk if needed for desired consistency.Turn the speed down to "stir" and mix for 2 minutes to eliminate big air bubbles.

Remove the cooled cakes from the pans. Place 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 30 minutes to set the crumb coat. Keep the bowl of remaining buttercream covered. Frost all over with a final, swirly layer of buttercream.

If you have any extra buttercream leftover, fit a piping bag with tip 1M, fill with the buttercream, and pipe a few swirls on top of the cake. Decorate with chocolate squares.