Preheat the oven to 350?F (178?C) and line a 12-muffin tin with greaseproof liners.

In a bowl, whisk together the flour, baking powder, and salt.

In a large mixing bowl, beat the butter and sugar until light and pale using an electric mixer (about 2-3 minutes). Scrape down the sides as needed.

Add the eggs one at a time, beating after each addition. Mix in half of the milk and the vanilla extract. Then add the key lime juice and zest, beating until well combined.

Incorporate half of the flour mixture on low speed until just combined. Add in the remaining milk, mix, and then add the rest of the flour mixture until just incorporated. Carefully fold with a spatula to avoid overmixing.

Divide the batter among the cupcake liners using an ice cream scoop, filling each just over halfway.

Bake for 20-25 minutes until tops spring back when pressed, and a toothpick inserted into the center comes out clean.

Remove from the oven and allow to cool in the pan until cool enough to handle, then transfer to a wire rack to cool completely.

Beat the cream cheese and butter with an electric mixer until smooth. Gradually add the powdered sugar while mixing. Incorporate the lime juice and zest, being careful not to overmix.

For a two-tone frosting, mix 2 tablespoons of frosting with green and yellow food coloring until lime green. Refrigerate for 30 minutes to firm it up for piping.

Place a piping tip of choice into a piping bag. Fill one side of the bag with green frosting and the other side with white frosting. Pipe onto the cooled cupcakes.