1. To make the lemon filling: Put all of the ingredients in a double boiler or in a heatproof bowl set over a saucepan of barely simmering water. Whisk constantly until the mixture becomes very thick, like jam, about 15 minutes. Remove from the heat, put a piece of plastic wrap directly on the surface of the curd, and refrigerate until completely chilled, about 2 hours.
2. Meanwhile, make the coconut cupcakes: Preheat the oven to 350°F. Generously butter and sugar twelve 4-ounce ramekins or line a 12-cup muffin tin with paper or foil liners; set aside.
3. Sift the flour into a large mixing bowl and stir in the coconut. Set aside.
4. Put the egg whites in the bowl of an electric mixer fitted with the whisk attachment and whisk at medium-high speed until frothy. With the machine running, slowly add 1/3 cup of the sugar and continue whisking until medium-soft peaks form. (When you lift the whisk from the mixture, a peak will form and the top of the peak will fall back down.) Transfer the whipped whites to another large bowl.
5. Put the egg yolks and lemon zest in the electric mixer bowl and whisk at medium speed until the yolks break, then add the salt and the remaining 2 tablespoons plus 2 teaspoons sugar and continue whisking until the yolks are pale yellow, thick, and doubled in volume, about 10 minutes.
6. Add the vanilla to the yolk mixture and whisk until well incorporated. Add one-third of the whipped whites to the yolks and stir well. Carefully fold in the remaining whites just until incorporated, trying to make sure they do not lose volume. Fold in half the flour mixture, then fold in the remaining flour until no traces of flour remain.
7. Divide the batter among the ramekins or muffin cups. If using ramekins, put them on a baking sheet. Bake for 4 minutes, then rotate the pan and bake until a tester comes out clean and the tops are puffed and golden brown, about 5 minutes more. Remove from the oven and cool for 5 minutes in the ramekins or pan, then unmold and cool completely on a rack. The cakes will deflate as they cool.
8. Transfer the chilled lemon filling to a pastry bag fitted with a small plain tip. Poke a hole in the top of each cupcake with the tip of the pastry bag, and slowly pipe in the curd until it begins to squirt out the opening. You will be able to feel when the cupcake is filled. Or, if you do not have a pastry bag, use a small paring knife to poke a hole in the top of each cupcake. Put the lemon curd in a large Ziploc bag, snip off the tip of one of the corners to make a small hole, and use the bag to pipe the curd into the cupcakes. Serve the same day.