Begin by making the lime curd because it needs to chill in the refrigerator in order to set. In a small sauce pan, combine the eggs, sugar, salt, lime juice and zest. Whisk until smooth. Heat over medium until the mixture begins to bubble.
Then stir constantly until the mixture thickens. Remove from the heat. Set out a medium sized bowl and strain the curd mixture - this should capture most of the lime zest and any potential cooked egg pieces. Add in the butter and food coloring and whisk until the butter has melted and the mixture is smooth. Cover with plastic wrap so that it is touching the top of the curd to prevent a film from forming during the chill time. Allow to cool in the fridge for about an hour in order to set up.
While making the curd and/or during the chill time, make the cupcakes. Follow directions on the box to make 24 as directed. Set on a cooling rack to come to room temperature before filling.
Once the cupcakes have cooled and the curd has set up, it’s time to decorate. Transfer some store bought frosting to a piping bag (or ziploc bag) fitted with a large round tip (or just a hole cut in the end), set aside. Transfer the chilled curd to a piping bag (or ziploc bag), set aside.
Scoop out the center of each cupcake. Cut a hole in the tip of the curd bag and carefully pipe enough in the void of each cupcake to fill but not overflow. Gently replace the reserved cupcake piece on top of the lime curd to cover the curd and give the frosting something to grab on to.
With the frosting bag, begin on the outside edge of the cupcake and squeeze the frosting carefully to go around the cupcake and then spiral inwards and up to cover the curd and create a mound. Repeat with the remaining cupcakes.