For the Cupcakes
Center a rack in the oven and preheat the oven to 350 degrees F. Prepare 2 muffin pans with cupcake liners.

Sift together the flour, baking powder and salt.

Whisk together the milk and egg whites in a medium bowl.

Put the sugar and lime zest in a mixer bowl or another large bowl and rub them together with your fingers until the sugar is moist and fragrant. Add the butter and working with the paddle or whisk attachment, or with a hand mixer, beat at medium speed for a full 3 minutes, until the butter and sugar are very light.

Beat in the lime juice, then add one third of the flour mixture, still beating on medium speed.

Beat in half of the milk-egg mixture, then beat in half of the remaining dry ingredients until incorporated. Add the rest of the milk and eggs beating until the batter is homogeneous, then add the last of the dry ingredients. Finally, give the batter a good 2- minute beating to ensure that it is thoroughly mixed and well aerated.

Divide the batter into the cupcake liners. Bake for 20-25 minutes, or until the cupcakes are well risen and springy to the touch. Transfer to cooling racks and cool for about 5 minutes, then remove cupcakes from the pans and cool completely. Once cupcakes have cooled, brush the top of each cupcake with a 1/2 tsp of tequila.

For the Frosting
Add butter to the bowl of a stand mixer, beat until light.

Add half the tequila, lime zest and salt, mix.

Add powdered sugar 1/2 cup at a time, mixing well and scraping down sides as necessary.

Continue to add sugar and tequila as necessary.