Preheat the oven to 350 degrees. Line 20 to 22 muffin cups with paper liners and set pans aside.

Place all cupcake ingredients in a large mixing bowl and beat with an electric mixer on medium speed for 2 minutes. Divide batter between the prepared muffin cups and bake 14 to 18 minutes, until a cake tester inserted into the center of the cupcakes comes out clean.

While your cupcakes are in the oven, prepare the tequila-lime syrup.

Heat the lime juice in the microwave for about 30 seconds. Stir in the sugar until it dissolves, then stir in the tequila.

Remove cupcakes from the oven and use a toothpick to poke several holes about 1 inch deep into each of the cupcakes. Use a pastry brush to brush the syrup over the cupcakes. Carefully remove cupcakes from the pans and cool completely on a wire rack.

Once cupcakes are cool, make the frosting. Beat butter until very smooth, then add 3 1/2 cups powdered sugar, 1 tablespoon milk, tequila, cointreau, lime zest, and salt and mix to combine. Scrape down the sides and bottom of the bowl, then turn mixer to high and beat 3 to 4 minutes, until very fluffy. Add additional powdered sugar if frosting is too thin and additional milk if frosting is too thick. Frost cupcakes and sprinkle with salt and lime zest as desired.