I adapted the recipe and added my comment in red.
1 cup butter (2 sticks)
2 cups sugar
4 large eggs
3/4 cup Mountain Dew
1/4 cup of lemon and lime juice (juice from 1 lemon and 1 lime)
zest from 1 lemon and 2 limes (separate into 2 piles, one for butter cream) (see note below on the lemon and limes)
1 1/2 teaspoons lemon extract
2-3/4 cups all-purpose flour
1-1/2 teaspoons baking powder
1/4 teaspoon salt
4-5 cups powdered sugar
2 tablespoons Mountain Dew
Juices from half of 1 lemon and half of 1 lime.
Lemon and lime zest (rest of zest from cupcake recipe. Or zest from half of 1 lemon and 1 whole lime)
1 teaspoon lemon extract
neon green food coloring (yellow and blue if you don't have green)(did not use)
With an electric mixer beat the butter at medium speed for about 30 seconds and add the lemon and lime zest and beat until light and fluffy.
Add the eggs one at a time, Scrape sides of bowl in between additions.
Whisk together the flour, baking powder, and salt in a medium size bowl.
Add the flour mixture to the butter mixture. Mix with your electric mixture on low for about 5 seconds
Add the Mountain Dew, lemon/lime juice, and lemon extract . Mix to combine.
Scoop into cupcake papers about two-thirds full.
Bake for 18-20 minutes until a cake tester comes out clean.
Sift powdered sugar into a bowl or onto parchment paper
In a medium size bowl beat butter at medium-high speed until creamy. Add half of the sugar, turn mixer on low for 5 seconds.Add the Mountain Dew, zest, and lemon extract . Beat until combined.
If you want to add food coloring you can add it at this point.
Gradually add remaining sugar until you get to the consistency and sweetness you like.
**Lemon and Lime note: used 1 lemon and 2 limes zest in the batter, for the butter cream I added the same amount.