Preheat oven:

Heat the oven as directed on the white cake mix box.

Mix cake batter:

Mix the cake batter per box instructions, substituting 1/2 cup of the water with 1/2 cup lime juice.

Line muffin cups:

Place liners into 24 muffin cups.

Fill cupcake liners:

Pour batter into each liner until about half full.

Bake cupcakes:

Bake for 16–18 minutes, until a toothpick inserted in the center comes out clean.

Cool cupcakes:

Take cupcakes out of the pan and let cool fully on a wire rack before applying frosting.

Cream butter:

In a large bowl, cream 1 cup softened butter until smooth.

Add lime juice and coloring:

Add 6 tbsp lime juice and 1 drop green food coloring; mix until combined.

Add powdered sugar:

Gradually add 6 cups powdered sugar, 1 cup at a time, beating thoroughly after each addition until fully blended.

Add lime zest and whip:

Stir in 2 tsp lime zest and whip until evenly incorporated and the frosting is light and fluffy.

Frost cupcakes:

Pipe or spread the frosting onto the cooled cupcakes using a piping bag or knife.

Serve or store:

Serve right away or keep in an airtight container.