1. Prepare the dough:
In a food processor, mix together the flour, sugar, and butter until the mixture becomes crumbly.
Add the egg yolk and continue to mix.
On a floured work surface, knead the dough.
Cover and refrigerate for 30 minutes.
2. Grease a 12-cup muffin tin.
Roll out the dough to a thickness of 4 mm and cut out 12 discs with a diameter of 8 cm.
Line the muffin tin cups with the dough discs. Prick the bottom with a fork and refrigerate for 30 minutes.
3. Preheat the oven to 200°C (400°F).
Bake the muffin tin for 10 minutes.
Let it cool.
4. Meanwhile, mix the sugar and cornstarch in a small saucepan.
Gradually pour in the orange juice and water. Heat while stirring until the mixture boils and thickens.
Reduce the heat and let it simmer for 1 minute, stirring constantly. Remove from heat and add the orange zest, butter, and egg yolks. Let it cool for 10 minutes.
5. Divide this mixture among the tartlet shells and refrigerate for 1 hour.
6. Increase the oven temperature to 240°C (475°F).
In a bowl, beat the egg whites until soft peaks form.
Gradually add the sugar and continue to whisk until dissolved.
7. Scrape the surface of the tartlets with a fork.
Using a star-tipped piping bag, pipe rosettes of meringue onto each tartlet. Bake for 3 minutes until lightly browned.