Place the egg yolks in a bowl, and whisk them until they're well blended.  Add the sweetened condensed milk, 3 egg yolks, sugar, and key lime juice to a heavy bottom saucepan. 

Cook over medium-low heat, stirring constantly, until the mixture thickens, about 15 minutes.  The filling will continue to thicken as it cools. Place in the refrigerator until time to fill the cupcakes.

WHITE CHOCOLATE CURLS - 1 bar of White chocolate, and a carrot peeler is what you'll need to make the curls that go on top of the cupcakes.  Start with small strokes on the white chocolate bar to make small tight curls. 

If you prefer bigger curls, just continue down the bar with the peeler.  You can make these while the cupcakes are baking and cooling.  Place curls on parchment paper and place in the refrigerator until time to top cupcakes.

In the mixing bowl of a stand mixer, combine the butter, sugar, eggs, Key Lime juice, lime zest, milk, and sour cream, and mix until smooth. 

In a separate bowl, sift the two flours until they look like fine powder.  Add the baking powder and salt, and mix together. 

Add the flour mixture to the mixing bowl a little at a time, and mix between additions, until all ingredients have been blended. 

Mix on high for 1 minute.  Place cupcake liners in muffin tins, and fill the liner 3/4 full with the cupcake batter.  

Bake on 350 degrees for 15 - 20 minutes, or until the cupcakes are golden brown on top, and a toothpick inserted in the middle comes out clean.  Remove from the oven, and cool on wire racks.

In the mixing bowl of your stand mixer, blend the cream cheese and butter until smooth.  Add the key lime juice, vanilla and lime zest, and mix until blended. 

Add the powdered sugar in small intervals, and blend between additions.  If the frosting seems too thin, add a bit more powdered sugar.  If it's too thick, add bit more key lime juice, 1 teaspoon at a time, until desired consistency is achieved.  Frosting should be thick enough to pipe onto cupcakes.

Place the mixing bowl in the refrigerator, covered with foil until time to frost the cupcakes.

When the cupcakes have completely cooled, remove the center of the cupcake with the corer. 

When centers are removed, slice a few Key Limes in thin slices, and place a slice in the center of each cupcake, then fill the center with the KEY LIME FILLING. 

Once all the cupcakes have been filled with filling, remove the frosting from the refrigerator, and place in a pastry bag with the star tip. 

Pipe frosting on cupcakes, working from the base, swirling around the cupcake and to a point. 

Top with white chocolate curls. 

Cover, in a single layer cupcake container, and place cupcakes in the refrigerator for at least 2 hours before serving. Chill overnight for best results.