In a small bowl, combine milk with vinegar and let sit for 5-10 minutes to thicken.

In a large mixing bowl, whisk together gluten-free flour, cocoa powder, baking powder, baking soda, granulated sugar, and salt.

In a separate bowl, beat together softened butter and milk mixture. Add vanilla extract and mix until smooth.

Gradually stir in hot brewed coffee into the wet mixture until evenly combined.

Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans. Pour batter evenly into pans and bake for 35 minutes or until a toothpick comes out clean.

Allow cakes to cool in pans for 15-20 minutes before transferring to a wire rack.

In a large bowl, beat together softened butter, cocoa powder, and powdered sugar until fully incorporated.

Layer the cooled cakes on a serving platter, spreading frosting in between layers and covering the top and sides.