Combine flour, granulated sugar, and salt in a food processor. Pulse in cold, cubed butter until the mixture resembles coarse crumbs. Add egg yolk and pulse again; incorporate cold water if too dry. Knead briefly, wrap in plastic, and chill for 30 minutes.

Preheat the oven to 350°F (175°C). Roll out the chilled pastry on a floured surface until about 1/8 inch thick. Cut out circles and press into tartlet molds. Prick the bottoms with a fork, bake for 12-15 minutes until golden brown. Cool completely.

Whisk together lemon juice, lemon zest, granulated sugar, and eggs in a heatproof bowl. Place over a gently simmering double boiler, whisking constantly for about 8-10 minutes until thickened. Remove from heat and whisk in butter until smooth. Strain the curd through a sieve and chill for at least 1 hour.

Spoon or pipe the lemon curd into each cooled tartlet shell. Garnish with powdered sugar, fresh berries, or whipped cream if desired.

Serve fresh for the best flavor. Store in the refrigerator for up to 3 days, adding toppings just before serving.