In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
Pour milk into a 1-cup measuring cup and add 2 tablespoons of fresh-squeezed lime juice. Stir to incorporate and set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter and sugar on medium-high speed until light and fluffy, about 2 minutes.
With the mixer on low, add in eggs one at a time, beating well after each.
Slowly add in the flour mixture in three batches, alternating with the milk mixture, starting and ending with the flour. Mix until just incorporated and make sure speed is always on low.
Divide batter evenly in a muffin pan. Bake for 18-24 minutes at 350F or until a toothpick inserted into the center comes out clean.
When done baking, remove cupcakes from oven and brush with tequila. After about 5 minutes, transfer cupcakes to a wire rack to completely cool before frosting.
For the Buttercream:
In the bowl of a stand mixer fitted with paddle attachment, add butter. Beat until light and fluffy, about 3 minutes.
Turn mixer off and add in confectioner's sugar. Turn mixer on low and slowly add in fresh-squeezed lime juice, one tablespoon at a time.
Mix until thick and creamy.
Make sure to add the tequila to the cupcake while it is still warm! I have found the best way to do this is to brush it on with a pastry brush, but you can also use a small spoon and pour it over.
To decorate the cupcake, add frosting to a pastry bag fitted with an open-star tip. Pipe out frosting onto cooked cupcake and top with sanding sugar. If you want to add salt, sprinkle just a couple granules over the top cupcake.
Cupcakes last about 3 days but are best served immediately.