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, 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.

Place one of the cooled cakes on a cake board or cake pedestal.

Fit a piping bag with tip 6B and fill with the buttercream. Pipe the buttercream onto the cake, then place the second cake layer on top. Repeat piping and stacking the cake layers, then pipe a final layer of buttercream on top of the cake.

Pile the buttercream with marshmallows and dust lightly with powdered sugar.

If you don't want to make a naked cake, the frosting recipe yields enough to completely frost and cover the cake with buttercream.