Preheat the oven to 375°F (190°C).

1. In a food processor, combine the flour, butter, and sugar. Pulse until the mixture resembles coarse crumbs.
2. Add the egg yolk and pulse again until combined. Slowly add cold water, 1 tablespoon at a time, until the dough comes together.
3. Turn the dough out onto a lightly floured surface and knead it a few times until smooth. Roll the dough out to about 1/8 inch thick and cut into circles that fit into the cups of a mini muffin tin.
4. Gently press the dough into the muffin tin and trim any excess dough around the edges.
5. Blind-bake the tart shells by placing a piece of parchment paper over the dough and adding pie weights or dried beans. Bake for 8-10 minutes until the edges are lightly golden.
6. While the tart shells are cooling, make the lemon filling. In a medium saucepan, whisk together lemon juice, lemon zest, sugar, and eggs over medium heat. Cook, stirring constantly, until the mixture thickens (about 5-7 minutes).
7. Remove from heat and whisk in the heavy cream and butter until smooth and creamy.
8. Pour the lemon filling into the cooled tart shells, filling each about 2/3 full.
9. Bake the filled tarts for an additional 10-12 minutes, or until the filling is just set.
10. Allow the tarts to cool completely in the tin before transferring them to a serving platter.