Preheat oven to 350 degrees Fahrenheit and prepare three 8 inch cake pans by lining the bottom of each with parchment paper. Then, grease and flour the pans.
In a large bowl, whisk together flour, both sugars, espresso powder, cocoa powder, baking powder, baking soda, and salt.
Add oil, buttermilk, eggs, and vanilla. Mix to incorporate. Add hot water and stir until just combined.
Divide batter evenly among prepared cake pans. Tap each pan gently on the counter to remove excess air bubbles, and bake at 350 degrees for 30-34 minutes. A toothpick inserted into the center of each cake should come out with a few moist crumbs attached.
Remove cakes from oven and let cool in pans for about 15 minutes. Then, turn out each cake on to a wire cooling rack and let cool completely.
While cake is cooling, make the skyscrapers. Starting with one bag, melt the candy melts according to the instructions on the package.
Line a large cookie sheet with parchment paper. Pour the melted candy melts on top of the parchment paper and use an offset spatula to spread into a large rectangle. Don't spread it too thin, aim for about an 1/8th inch thick or so. The height of the rectangle should be slightly taller than how tall you want your skyscrapers to be.
Top with sprinkles and let set at room temperature until slightly hardened, but still pliable (candy should no longer be sticky to the touch, but should still have a slight bend to it so it doesn't crack when being cut).
Using a large, sharp knife, cut the rectangle into skyscraper shapes, using inspiration from your favorite city. Straight lines and long edges make easy work, but feel free to get creative!
Carefully transfer cut skyscrapers to another parchment paper lined baking sheet and chill in the fridge until hardened completely.
Repeat with remaining candy melts until you have enough skyscrapers to surround your entire cake (I used about 2 1/2 bags of candy melts for skyscrapers about 2x as tall as my cake).
Using an electric mixer, beat butter on medium-high speed until light and fluffy.
Gradually add powdered sugar (about 1 cup at a time), then cocoa powder, mixing on low speed until incorporated after each addition.
Add salt and vanilla and mix to combine. Add milk, then continue to beat until completely incorporated and frosting is light and fluffy.
If necessary, adjust consistency as desired by adding more powdered sugar for a thicker frosting, or more milk to make it thinner.
Level each cake layer by slicing off the rounded domes with a sharp serrated knife or a cake leveler.
Place the first layer on top of a cake board, using a small amount of buttercream to secure it.
Top with about 1 cup of frosting and smooth into an even layer using an offset spatula. Top with second layer and repeat. Place final layer on top of the cake, cut side down.
Frost the top and sides of the cake with a thin layer of frosting and chill the cake for about 20 minutes.
Finish frosting the cake, reserving about 1/2 cup of frosting to secure the skyscrapers.
Remove skyscrapers from fridge and spread a little buttercream on the back (up to the height of the cake). Place skyscrapers on the sides of the cake and press gently to secure.
Keep cake chilled to prevent skyscrapers from melting.
* Make your own buttermilk by adding 1 tablespoon white vinegar or lemon juice to a scant 1 1/4 cup of whole milk.
**I used black candy melts, but you could use whichever color you prefer. If you'd like, you could also try this with melted and tempered chocolate!