Preheat oven to 350 degrees F and grease two 8-inch round cake pans. (you can also use for cupcakes) If your cake pans aren't nonstick, you may want to add parchment paper as well.
In a small bowl, combine the milk and vinegar and set it aside.
Sift together the granulated sugar, all-purpose flour, cocoa powder, baking powder, baking soda, and salt into a stand mixer bowl.
In a mixing bowl, add in the eggs, prepared milk mixture, oil, and vanilla. Mix until the wet ingredients are combined, scrape the bowl if using a mixer.
Add the dry ingredients to the wet ingredients until just combined. Next, add in the hot coffee and then mix again until just combined. The batter will have a runny consistency.
Bake cakes for 30-35 minutes, or until toothpick inserted shows minimal crumbs. If making cupcakes, bake for 12-15 minutes or until a toothpick inserted shows minimal crumbs.
Place the pans onto a wire rack and cool for at least 20 minutes before removing the cakes.
Chocolate Buttercream
Add the butter to a large mixing bowl or the bowl of a stand mixer. Beat on medium speed until it’s smooth, creamy, it should be pale in color, about 2 minutes.
In a separate bowl, sift together the powdered sugar and cocoa powder to remove any clumps.
With the mixer on low speed, slowly add the sifted sugar and cocoa into the butter a little at a time. Mix just until combined, scraping down the sides of the bowl as needed.
Pour in the milk and vanilla extract, then increase the mixer speed to medium-high. Beat until the frosting is light, fluffy, and spreadable, about 2 to 3 minutes.
Check the consistency: if the frosting is too thick, mix in another splash of milk. If it’s too thin, add a little more powdered sugar until it holds its shape.