Preheat oven to 350 F (175 C).

In a medium-sized mixing bowl or bowl of a stand mixer, mix together sugar, cake flour, baking powder, baking soda, salt, and zest.

Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.

In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.

Add the egg mixture to the flour mixture and beat on medium speed until just combined.

Slowly add milk and key lime juice and mix on low speed until just combined. The batter will be liquid. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)

Fill cupcake liners just over ½ full.

Bake for 14 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. These cupcakes will have flat tops, so do not look for doming as a sign of doneness. If they are not done, test again in two minutes. If they are still not done, test again in another two minutes.

When the cupcakes are done, remove them immediately from the tins and leave them on a cooling rack (or just on your counter if you don't own a cooling rack) to cool.

Beat butter on high speed with an electric mixer for about three minutes until light and airy.

Mix in powdered sugar a little bit at a time.

Add salt and sweetened condensed milk and continue to beat for another minute. If desired, add more powdered sugar to make the buttercream frosting stiffer.

Pipe or spread onto cooled cupcakes.