Preheat oven to 350F. Grease and flour 3 8-inch round baking pans and set aside (for good measure I also line the bottom with parchment paper but you don't have to).
Using the bowl of your stand mixer (or a large bowl if you're using a handheld electric mixer), sift sugar, flour, cocoa powder, baking powder, baking soda and salt.
Add eggs, milk, oil and vanilla. Beat on low-medium speed until well incorporated (about 2 minutes). Add coffee and stir until combined. The batter will be thin.
Transfer the batter into your prepared pans and bake for 30 minutes or until a toothpick inserted in the centre comes out with minimal crumbs.
Cool in the pans for 10 minutes then on a wire rack to cool completely.
For the Dark Chocolate Fudge Frosting:
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat unsalted butter on medium speed until light and creamy (about 5 minutes using a stand mixer), scraping the bottom and sides of your bowl as you go.
Switch to low speed and add confectioner's sugar, cocoa powder, coffee, sour cream, vanilla and salt until combined (about a minute). Switch to medium speed and beat for another 2 minutes.
Add the melted chocolate and beat until smooth (about a minute).
Trim the tops of your cakes if they are uneven using a serrated knife or a cake leveller.
Take one and place on a platter or cake turntable, smooth side down (trimmed side up). Evenly cover the top with frosting. Repeat with the 2nd and 3rd layers. For your 3rd layer, remember to place it smooth side up (trimmed side down) so that you'll have an even top.
Finish the cake by also covering the sides of your cake with frosting. To help even out the frosting on top and the sides, use a bench scraper and gently scrape away excess frosting.