Step 1: Make the Crust

1. Prepare the Crust Dough: In a food processor, pulse together the flour and powdered sugar. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs. Add the egg yolk and pulse again until it comes together. If the dough is too crumbly, add a tablespoon of ice water at a time until it forms a dough.
2. Chill the Dough: Turn the dough out onto a floured surface, shape it into a disk, and wrap it in plastic wrap. Chill the dough in the fridge for at least 30 minutes.

Step 2: Prepare the Tartlet Shells

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Roll out the Dough: On a lightly floured surface, roll the chilled dough out to about 1/8-inch thick. Carefully cut it into rounds large enough to fit into your tartlet pans.
3. Fit the Dough into Pans: Gently press the dough rounds into the tartlet pans, making sure the dough is evenly distributed along the bottom and up the sides. Use a fork to prick the bottom of the dough to prevent it from puffing up while baking.
4. Bake the Crust: Place the tartlet pans on a baking sheet and bake for 10-12 minutes, or until the crust is golden brown. Let the crusts cool completely before adding the lemon filling.

Step 3: Make the Lemon Filling

1. Prepare the Lemon Filling: In a medium saucepan, whisk together the lemon juice, lemon zest, sugar, and eggs until smooth. Place the saucepan over medium heat and cook, whisking constantly, until the mixture thickens (about 5-7 minutes).
2. Finish the Filling: Remove the saucepan from the heat and whisk in the heavy cream, butter, and a pinch of salt until the butter is fully melted and the mixture is smooth.
3. Strain the Filling: For a smooth texture, strain the lemon filling through a fine mesh sieve into a bowl to remove any lumps or curdled egg bits.

Step 4: Assemble the Tartlets

1. Fill the Tartlet Shells: Once the crusts are cooled, spoon the lemon filling into each tartlet shell, filling them to the top.
2. Chill: Place the tartlets in the refrigerator for at least 2 hours, or until the filling is set.

Step 5: Garnish and Serve

1. Garnish: Before serving, garnish each tartlet with fresh berries, a dollop of whipped cream, or a light dusting of powdered sugar for a touch of sweetness.
2. Serve and Enjoy: Serve chilled, and enjoy the delightful combination of crisp crust and tangy, creamy lemon filling.