Preparation:

Step 1: Preheat the Oven

Preheat your oven to 350°F (175°C). Line a muffin tin with cupcake liners and set aside.

Step 2: Prepare the Cupcake Batter

1. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
2. In a large mixing bowl, beat the butter and sugar together until light and fluffy.
3. Add the eggs one at a time, mixing well after each addition.
4. Stir in the sour cream (or Greek yogurt), key lime juice, key lime zest, and vanilla extract.
5. Gradually mix in the dry ingredients, alternating with the milk, until well combined.
6. If desired, add a few drops of green food coloring to enhance the lime appearance.

Step 3: Bake the Cupcakes

1. Divide the batter evenly among the prepared cupcake liners, filling each about 2/3 full.
2. Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean.
3. Remove from the oven and let the cupcakes cool completely on a wire rack.

Step 4: Prepare the Key Lime Filling

1. In a small saucepan over medium heat, whisk together the key lime juice, sugar, and egg yolks.
2. Cook, stirring constantly, until the mixture thickens (about 5 minutes).
3. Remove from heat and stir in the butter until melted and smooth.
4. Optional: Stir in cornstarch to thicken the curd.
5. Let the filling cool completely before using.

Step 5: Fill the Cupcakes

1. Once the cupcakes have cooled, use a small knife or cupcake corer to remove a small section from the center of each cupcake.
2. Spoon or pipe the key lime filling into the hollowed-out centers.

Step 6: Make the Frosting

1. In a large bowl, beat the cream cheese and butter together until smooth and creamy.
2. Gradually add powdered sugar, mixing until well combined.
3. Stir in the key lime juice and vanilla extract.

Step 7: Frost and Decorate

1. Pipe or spread the frosting over the filled cupcakes.
2. Garnish with key lime zest, sprinkles, or a small slice of key lime.
3. Chill the cupcakes for at least 30 minutes before serving to set the frosting.