Making the Lemon Curd

1. In a saucepan, whisk the egg yolks, whole egg, and sugar until smooth and lighter in color.
2. Add lemon zest, lemon juice, and a pinch of salt.
3. Turn the heat to medium-low and keep whisking until thick like warm pudding.
4. Once thick, take it off the heat and whisk in the butter pieces until melted and glossy.
5. Strain it through a fine mesh strainer into a bowl, cover with plastic wrap pressed to the surface, and chill for at least 1 hour.

Baking the Meringues

1. Preheat your oven to 225°F (107°C) and line two baking sheets with parchment paper.
2. Beat egg whites with a pinch of salt until foamy, then add the sugar slowly, one spoonful at a time.
3. Continue beating until the mixture is glossy and holds stiff peaks, then sprinkle in cornstarch and add vinegar and vanilla.
4. Spoon or pipe small swirls onto the parchment and create a dip in the center for the curd.
5. Bake for 60 to 75 minutes, then turn off the oven and leave them to cool inside for at least 30 minutes.

Assembling the Dessert

1. Ensure the meringues and lemon curd are both cool. Spoon 1 teaspoon of lemon curd into the center of each meringue.
2. Add desired toppings like berries, a mint leaf, or zest just before serving.