Measure 4¼ cups of powdered sugar in bowl, set aside.
Add room temperature to slightly cool butter to stand mixer with the paddle (you can use a hand mixer but you will need to add beating time). Beat on high for about 2 minutes until butter turns lighter in color and starts to make a slapping sound against the bowl.
While beating on low, add ½ cup of powdered sugar to the butter. Continue until all of the powdered sugar is added. The mixture will be very clumpy.
Add the margarita mix, tequila, salt, lime juice from 1 lime, and a pinch of lime zest. Beat on high for about 4 minutes. The mixture will start to smooth and expand.
Add neon green food coloring and continue to beat on high for another 1 minute. Scrape sides of the bowl with a spatula to make sure all of the sugars are mixed and food coloring is consistent.
Decorate your cupcakes, cake, or cookies!
Notes
Once completed, if there are too many air bubbles, mix with a spatula to release.
You can make the buttercream ahead of time (or store if there is extra). Wrap tightly in plastic wrap and refrigerate for up to 1 week or freeze for up to 3 months. When you want to use it again, remove and let it come to room temperature. Whip with a stand mixer for 3-4 minutes until smooth again.
This recipe will decorate approximately 24 cupcakes.