Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans with gluten-free flour, or line the bottoms with parchment paper.
In a medium bowl, whisk together the gluten-free flour blend, baking powder, baking soda, and salt. Set aside.
In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a small bowl, whisk together the sour cream and milk.
Alternately add the dry ingredients and the wet ingredients to the butter mixture, beginning and ending with the dry ingredients. Mix on low speed until just combined, scraping down the sides of the bowl as needed. Be careful not to overmix.
Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.