Preheat the oven to 350°F. Grease and line two 8 or 9-inch cake rounds.
In a large bowl, whisk together the 2 ⅓ cups gluten free flour, ¾ cup unsweetened cocoa powder, 1 cup granulated sugar, 1 cup brown sugar, 2 teaspoons baking soda, 1 teaspoon baking powder, and 1 teaspoon fine sea salt. In a medium bowl, combine the 3 large eggs, ½ cup oil, ½ cup sour cream, ½ cup whole milk, and 1 teaspoon pure vanilla extract. Slowly add the wet ingredients to the dry using a wooden spoon or rubber spatula - the batter will be thick. Stir in the 1 cup hot, fresh-brewed coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula.
Pour the batter into the prepared pans and rest it for 10-15 minutes if using gluten-free flour. Then place in the oven, and bake for 40-50 minutes, until the tops are set and a cake tester comes out clean. Remove from oven and cool in the pan set on a wire rack for 30 minutes, then carefully turn the cake out onto a cooling rack and cool completely before frosting.
Notes
CN's Tips
Rest the batter. Let the batter rest while the oven heats. All gluten-free flours need time to hydrate before baking, so make sure to let that batter rest for 10-15 minutes before baking it.
Refrigerate before frosting. This cake is quite tender, and I like to bake it well in advance of when I plan to frost it. If you can, let the cake rest overnight, well wrapped, in the fridge. This will make it so much easier to frost when you're ready. If you don't have time, no worries, but an hour or two in the fridge will really help. Just make sure the cake is fully cooled before frosting it, even if it's at room temperature!
Make it refined sugar-free - Use coconut sugar or maple sugar in place of the granulated sugar and brown sugar.
Make it dairy-free - Swap non-dairy milk and non-dairy yogurt for Greek yogurt and whole milk.
Make it egg-free - I haven't tried making this cake without eggs, but you could try using 3 flax eggs in place of the eggs.
Use room-temperature ingredients. Bring the ingredients to room temperature before using. This will help them better incorporate into the batter, resulting in a smoother, more velvety crumb to the cake.
Swap the coffee. If you don't drink coffee, try 1 cup of tea, or just use water.