Add the sour cream, milk, and vanilla extract. Stir until fully combined.
*Learn more about cake flour here: Why Use Cake Flour.
**Plain Greek yogurt or buttermilk can be substituted. If you don’t have buttermilk, you can substitute by stirring 2 teaspoons of lemon juice or white vinegar into 2/3 cups of regular milk. Allow the mixture to stand for 5 minutes, or until thickened and slightly curdled.
This recipe is based on my favorite Vanilla Cupcakes. Click here for more info and a step-by-step tutorial: Perfect Vanilla Cupcakes.