Preheat your oven to 350 degrees. Line two cupcake pans with paper liners.
Combine boxed cake mix with eggs, oil and water in a stand mixer. Beat on low speed until combined. Zest half of the lemon and half of the lime and add into the batter.
Fill each cavity of the cupcake pan ⅔ of the way full with the batter. Bake for 20-23 minutes or until cupcakes are golden.
Remove from the oven and let cool completely.
Once cooled, cut a circle out of the center of each cupcake and fill with a teaspoon of lime curd. Place the cut out piece back on top.
Beat butter in a stand mixer until smooth. Add in powdered sugar 1 cup at a time until fully combined. Then add in salt and milk until it reaches a smooth and pipeable consistency.
Transfer frosting to a piping bag fitted with a large star tip and pipe frosting onto cupcakes.
Decorate cupcakes with yellow and green sprinkles, if desired.
Egg, oil and water measurements are according to the box of cake I used. Check your boxed cake mix for what it says to include. Baked, unfilled and unfrosted cupcakes can be frozen once completely cooled. They'll last in the freezer for a few months.