To make the lemon curd, place ¼cup lemon juice, eggs, egg yolk and sugar in a small heavy-bottomed saucepan. Whisk together until thoroughly combined.
Place over low-medium heat and whisk occasionally until mixture begins to thicken. Do not let it simmer or boil.
Whisk in butter one piece at a time.
Continue to cook until mixture is thick enough to mound when dropped by a spoon (about 10 minutes).
Remove from heat, let cool to room temperature and refrigerate in airtight container at least overnight or up to one week.
To make the lemon syrup, stir sugar and water together in a small saucepan.
Bring to a boil over medium-high heat. Swirl pot around to help dissolve sugar.
Remove from heat and cool to room temperature.
Stir in 1/3 cup lemon juice. Lemon syrup is ready to use. Syrup may be refrigerated in airtight container for up to one week.
Place cupcakes on jelly roll pans. Pierce each several times with a bamboo skewer.
Brush lemon syrup over each one, then go over each one again for maximum absorption.
Place lemon curd in pastry bag fitted with tip.
Insert tip right down into the center of each cupcake. Squeeze to fill with lemon curd. As soon as the top of the cupcake begins to expand and crack, ease up on pressure—the cupcake has taken on as much filling as it can. (Aim for about 1 tablespoon filling per cupcake.) If any lemon curd is oozing out of the top, simply wipe it away before frosting. Cupcakes may be made up to this point early in the day of serving.
Frost cupcakes with meringue frosting, using a teaspoon to make whirls and swirls on top of the cupcakes—they should look like the tops of little lemon meringue pies. Cupcakes may be made up to this point early in the day of serving.
Right before serving, run cupcakes under a hot broiler until meringue just becomes tinged with color. Alternately, you may use a crème brûlée torch and simply wave it back and forth over the cupcakes to brown the meringue. Cupcakes are now ready to serve.