Preheat the oven to 350 degrees F. Spray two 8-inch cake pans with nonstick cooking spray and line the bottoms with parchment. Spray the parchment and dust with flour.
Whisk together the flour, cocoa, baking soda and salt in a large bowl.
Beat together the butter and sugar in a stand mixer until light and fluffy, 1 to 2 minutes. Add the eggs one at time and beat until incorporated. Add the sour cream and continue to beat until fully incorporated. Then add the vanilla.
Stir together the instant espresso powder with the hot water in a spouted measuring cup. With the mixer running, add the dry ingredients in two additions, alternating with the espresso.
Divide the batter between the cake pans and bake until a tester inserted in the centers comes out clean, 30 to 35 minutes. Cool the cakes in the pans on a rack for 10 minutes, then remove the cakes to the rack to cool completely.
Slice a thin layer from the top of each cooled cake with a serrated knife to remove the domed top. With a serrated knife, slice the layers in half horizontally to make 4 layers. Spread buttercream on the cut side of a layer and sandwich another layer on top. Repeat with the remaining frosting and layers, leaving the last top layer unfrosted.
Apply a thin crumb-coat of frosting to the entire cake and chill in the refrigerator at least 15 minutes to set. Use the remaining frosting to generously frost the cake.
Put the egg whites and sugar into a large bowl set over a double boiler. Whisk constantly until the sugar melts and the mixture is very thin and warm.
Pour the egg whites into the bowl of a stand mixer fitted with the whisk attachment and whisk on high speed until stiff peaks form, about 5 minutes. Turn to low speed and continue beating until cool, about 10 minutes.
Turn the mixer to medium speed and whisk in the butter in small pieces. (The mixture may look curdled before it comes together.) Add the vanilla extract. Re-beat the mixture as needed to keep it smooth as you frost your cake.