Step 1: Make the Tart Crust

1. In a bowl (or using a food processor), mix the flour and powdered sugar. Add the cold butter and work it into the flour until the mixture resembles coarse crumbs.
2. Stir in the egg yolk and cold water, one tablespoon at a time, until the dough comes together.
3. Form the dough into a disk, wrap it in plastic wrap, and refrigerate for 30 minutes.

Step 2: Bake the Tart Crust

4. Preheat your oven to 375°F (190°C). Roll out the dough on a floured surface to fit your tart pan.
5. Press the dough into the tart pan and trim the edges. Prick the bottom with a fork.
6. Line the crust with parchment paper and fill it with pie weights or dried beans. Bake for 15 minutes, then remove the weights and bake for another 10 minutes until golden. Let cool completely.

Step 3: Make the Lemon Cream Filling

7. In a heatproof bowl, whisk together eggs, sugar, lemon juice, lemon zest, and a pinch of salt.
8. Place the bowl over a saucepan of simmering water (double boiler method). Cook, whisking constantly, until the mixture thickens (about 10 minutes).
9. Remove from heat and gradually whisk in the softened butter until smooth and creamy.
10. Strain the lemon cream through a fine-mesh sieve to remove any lumps.

Step 4: Assemble the Tart

11. Pour the lemon cream into the cooled tart crust. Smooth the top with a spatula.
12. Refrigerate for at least 2 hours, or until set.

Step 5: Garnish and Serve

13. Before serving, garnish with powdered sugar, fresh berries, or whipped cream for an extra touch of elegance.