Prepare the Pie Crust: In a large bowl, combine the flour and sugar. Add the cold butter and use your fingers or a pastry cutter to work it into the flour until the mixture resembles coarse crumbs. Beat the egg in a small bowl and mix it into the dough. Add a splash of ice water, just enough to bring the dough together without making it sticky. Form the dough into a disc, wrap it in parchment paper, and chill for at least 30 minutes.

Roll and Bake the Crust: Preheat the oven to 175°C (350°F). Roll out the chilled dough on a lightly floured surface to about 3mm thick. Use the top of a large Weck jar to cut out circles from the dough. The circles should be slightly larger than your tart tins to ensure they fit properly. Gently press the dough circles into the tart tins, smoothing them into place and trimming any excess. Prick the bottom of each tart shell with a fork and place in the freezer for 10 minutes to prevent shrinkage. Line the shells with parchment paper and fill with baking weights or dried beans. Bake for 12–15 minutes, then remove the weights and bake for another 5–7 minutes until lightly golden. Let cool completely.

Make the Lemon Filling: In a medium saucepan, whisk together the eggs, yolks, sugar, lemon zest, and lemon juice. Place the saucepan over low to medium heat, whisking constantly. Cook until the mixture thickens and coats the back of a spoon, about 5–8 minutes. Avoid overheating to prevent scrambling. Remove from heat and whisk in the cubed butter until fully melted and smooth. Strain the curd through a fine mesh sieve for an extra silky texture.

Assemble the Tarts: Pour the warm lemon filling into the cooled tart shells, smoothing the tops. Let them set at room temperature for 15 minutes, then chill in the refrigerator for at least 2 hours before serving.