For the cupcakes:

Sift together flour, baking powder, and salt.

Cream butter and sugar for about 3 minutes until fluffy.

Add eggs one at a time, mixing between each addition.

Mix in vanilla, lime juice, tequila, and lime zest.

Alternate adding flour and buttermilk, starting and ending with flour.

Fill cupcake liners about 2/3 full.

Bake at 325°F (163°C) for 22-25 minutes.

Cool in the pans for 10 minutes before transferring to wire racks to cool completely.

For the buttercream frosting:

Combine egg whites, sugar, and salt in a metal bowl.

Place the bowl over a pot of simmering water (double boiler).

Whisk constantly until the sugar is dissolved and the mixture reaches about 160°F (71°C).

Remove from heat and place the bowl on a stand mixer fitted with the whisk attachment.

Beat on medium-high speed until stiff, glossy peaks form and the mixture is cool (about 8-10 minutes).

Gradually add the room temperature butter, 2 tablespoons at a time, while continuing to beat.

If the frosting looks liquidy, continue beating until it comes together.

If the frosting appears curdled, keep beating. If it doesn't thicken, refrigerate for 15-30 minutes and then whip again.

The frosting should be smooth and satiny.

Add the tequila and lime juice while the mixer is running.

Once the cupcakes are completely cool, pipe the frosting generously onto each cupcake.

For an extra margarita flavor, brush cooled cupcakes with tequila before frosting.

Rub lime zest into the green (or white) sugar until fragrant.

Sprinkle the flavored sugar over the frosted cupcakes.

Add a sprinkle of coarse salt, if desired.