In a large mixing bowl filled with the cake mix, add in the water, oil, sour cream (or sour cream powder), and eggs. Using a hand or stand mixer, mix on medium until well incorporated.
Add batter into the muffin tin until each is about 2/3 full. Bake for 18-20 minutes, or until a toothpick comes out clean. Allow to cool completely before frosting.
Slowly mix in the powdered sugar, 1 cup at a time, while adding in the vanilla and a tablespoon of the sweetened condensed milk in between each cup. Continue to mix for 1 minute or until creamy.