Preheat the oven to 350ºF and prepare three 6-inch or two 8-inch cake pans by spraying the sides with baking spray and fitting the bottoms with a parchment cake circle. If you have them, wrap the pans in dampened cake strips (it's optional though).
Add the vinegar into the whole milk, stir, and set aside. This will begin to curdle as you measure out the rest of the ingredients, which is a good thing! It means the right chemical reaction is happening.
In a large bowl, add all of the dry ingredients: gluten-free baking flour, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt. Whisk together and set aside.
In a separate medium bowl, add the vegetable oil, eggs, vanilla, and the vinegar-milk mixture. Whisk together until fully combined, then add these wet ingredients into the bowl with the dry ingredients and whisk until combined and uniform. Then, whisk in the hot coffee until combined. The batter will be very thin.
Pour the batter into the prepared cake pans and bake for 32-36 minutes, until a toothpick inserted comes out clean. Let the cakes cool in the pan for about 10 minutes before removing and transferring to a wire rack to cool completely to room temperature.