Grease and flour (3) 8” round cake pans. Line bottom of pans with parchment paper; set pans aside.
Using a medium mixing bowl, add flour, cocoa powder, baking soda and baking powder; stir until well combined. Set dry mixture aside.
In a separate bowl, add coffee, milk and vanilla extract; stir until well combined. Set wet mixture aside.
Using an electric mixer fitted with paddle attachment, add butter, sugar and salt; cream together on medium speed for 4-5 minutes, or until light and fluffy.
Add eggs one at a time, mixing fully after each addition.
Add ⅓ of the flour mixture to the bowl; mix until well combined.
Add ½ of the liquid mixture; mix until well combined.
Repeat process, alternating between dry ingredients and liquid ingredients; mix well after each addition. (Tip: Don't forget to scrape down the sides of the bowl with a spatula!)
Divide batter evenly into the prepared cake pans.
Bake for 24-26 minutes, or until a toothpick inserted into the center of each cake comes out mostly clean.
Remove cakes from oven; let cool for 10 minutes before transferring to a cooling rack. Let cake layers cool completely before frosting. (Tip: If time allows, refrigerating the cakes will make them easier to frost.)
For the Frosting
Fill a medium saucepan with 2” of water and place over low heat; bring to a light simmer.
Using a heat-proof medium bowl, add sugar, water, egg whites, cream of tartar and salt; mix with a hand-mixer for 30 seconds.
Place bowl over the saucepan of simmering water (bottom of the bowl should not touch the water); beat on high speed for 7 minutes, or until frosting is stiff.
Remove bowl from heat and add vanilla extract; continue beating for 1-2 more minutes. (Note: This frosting will set quickly, so be prepared to frost the cake right away.)