In a small bowl, whisk together the egg yolks, key lime zest, key lime juice, sugar and salt. Transfer to a small saucepan.

Cook the mixture, stirring occasionally, over low-medium heat until it is thick enough to coat the back of a spoon, 3 to 5 minutes. Stir frequently enough that you don't scramble the yolks. You want to cook them slowly.

Remove from heat and add the butter. Mix until smooth.

Transfer the curd to a small bowl and cover with plastic wrap.

Refrigerate until chilled, about 30 minutes.

Line 24 regular-sized muffin cups with paper liners.

In a large mixing bowl, combine the cake mix, key lime zest, key lime juice, and water.

Beat on low speed for 30 seconds to incorporate, then raise the speed to medium and beat until the mixture is light and fluffy, about 1 minute. Batter will be super fluffy.

Fill the cupcake liners 3/4-full with batter.

Bake at 350 degrees for 12-15 minutes, or until cupcakes are golden brown.

Cool completely, about 30 minutes.

In a medium bowl, whisk together the pudding mix and the milk.

Fold in the whipped topping and the lime zest.

Using the end of a wooden spoon (or your finger!), press an indentation into the middle of each cupcake, about 1 inch deep.

Fill the indentation with 1-2 tsp. of key lime curd.

Frost cupcakes with pudding frosting. Store in refrigerator.