Preheat oven to 350 degrees (F). Line 2 cupcake tins with paper liners; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, beat the butter, vanilla, and sugar on medium speed until light and fluffy, about 2 minutes. Add in the eggs, one at a time, beating well after each addition. On low speed, beat in the sour cream, then the milk. Turn mixer off. Add in the cake flour, baking powder, baking soda, and salt. Use a fork to lightly toss the dry ingredients together. Using a rubber spatula, gently fold the dry ingredients into the batter, mixing just until combined. Be sure not to over mix here or you cupcakes will be dense!

Spoon batter into prepared cupcake tin (filling 1/2 way each - not more or they'll overflow) and bake for 18-20 minutes. Cool completely before frosting.

In a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, beat the butter on medium-speed until completely smooth. Reduce the mixer speed to low and gradually add the confectioners sugar, no more than a 1/2 cup at a time. Once all of the sugar has been incorporated, pour in the milk and vanilla extract. Once all of the ingredients have been incorporated, increase the speed to medium-high and beat for 2 to 3 minutes, or until fluffy.

If the frosting appears too thin, add a little confectioners' sugar, one tablespoon at a time; if the frosting appears too thick, add a little more milk, one tablespoon at time.