Prepare lime curd: In a pot, bring 1/2 cup butter, sugar, lime zest, lime juice, cornstarch and salt to a boil, whisking constantly.
Whisk egg yolks in a heatproof bowl until light in color.
Whisk a small amount of the hot mixture into the egg yolks. Pour the egg yolk mixture back into the pot of hot liquid while whisking thoroughly.
Heat mixture at a low simmer, whisking constantly to prevent sticking and burning, for 3 to 4 minutes, until mixture thickens and holds the marks of the whisk.
Pour into a bowl, cover with plastic wrap and chill.
Prepare the cupcakes: Preheat the oven to 350 degrees. Place 3/4 cup coconut in a shallow baking pan and toast coconut as the oven preheats. Stir coconut every few minutes and remove from the oven once it is toasted and golden brown. Set aside.
Place cake mix in a large mixing bowl (or in the bowl of a standing mixer). Add cream of coconut, eggs, buttermilk, vanilla and coconut flavoring and mix well, scraping down sides of bowl once or twice. Briefly chop the shredded coconut in a food processor, then fold into the cake batter.
Place cupcake liners into about 20-24 cupcake tins (you can bake cupcake in batches if necessary). Fill cupcake tins about 2/3 of the way to the top. Place cupcakes in the oven and bake cake for about 20 minutes, or until they have rised and spring back lightly to touch. Remove from the oven and set aside to cool for several minutes, then remove cupcakes from pan and let them cool on a rack.
Once cupcakes are cool, fill them with lime curd: Place lime curd in a piping bag with a round metal tip. Use tip to poke a hole in the top of each cupcake, and gently squeeze as much line curd as you can into the center of the cupcake. Alternatively, you can cut a small disk out of the top of the cupcake with a sharp knife, add the lime curd to the center of the cupcake, then replace the disk before frosting cupcakes. (see how to fill cupcakes with frosting )
To make the meringue frosting, place egg whites in the (very clean) bowl of an electric mixer, and attach the wire whisk.
Place the sugar, 1/3 cup water, corn syrup, and pinch of salt in a small saucepan, and bring to a boil.
When the temperature of the sugar syrup reaches about 240 degrees Fahrenheit (115 degrees Celsius), or after about 3 minutes, turn on the mixer and start beating the egg whites. When the temperature reaches 248 degrees Fahrenheit (120 degrees Celsius), remove the syrup from the heat. The total cooking time should be about 4 minutes, and the sugar syrup should make thin threads if you drizzle it off of a fork, and will have thickened slightly.
The egg whites should be forming stiff peaks. Slowly and carefully pour the syrup down the side of the mixer bowl, into the beaten egg whites. Continue to beat the meringue until it cools, about 5 minutes. Fold in 1 teaspoon vanilla or coconut flavoring if desired.
Decoratively spread the meringue frosting on the tops of the cupcakes, and sprinkle tops with reserved toasted coconut.
Store cupcakes in the refrigerator.