Preheat the oven to 350F. Line with parchment paper and grease two 8-inch round cake pans with butter and dust with gluten-free flour. Set aside.
In a medium bowl, add the gluten-free flour, cornstarch, baking powder, baking soda, and salt. Whisk until combined. Set aside.
In a large bowl, add the granulated sugar and vegetable oil. Beat with an electric mixer for a few minutes, until combined.
Add the eggs, vanilla extract and apple cider vinegar. Mix until smooth.
Add 1/3 of the dry ingredients to the cake batter. Mix until combined.
Combine the sour cream and milk in a measuring cup or bowl. Add 1/2 of this sour cream mixture to the cake batter. Mix until combined.
Repeat with 1/3 of the dry ingredients, the remaining 1/2 of the sour cream mixture, and finish with the last 1/3 of the dry ingredients, mixing in between each addition.
Pour the vanilla cake batter into the prepared cake pans.
Bake at 350F for 35-40 minutes, until a toothpick inserted into the center comes out clean.
Let the cake cool for 10 minutes before inverting from the pans onto a wire cooling rack to finish cooling.
Cool completely before frosting and serving.