Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.

In a small bowl, whisk together milk and white vinegar. Let sit for 5 minutes to curdle.

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

In a separate large bowl, cream together softened butter and granulated sugar until light and fluffy.

Beat in eggs one at a time, then stir in vanilla extract and the milk mixture.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix. Pour batter evenly into prepared pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cakes cool in pans for 10 minutes before inverting onto a wire rack to cool completely.