In a food processor, pulse together flour, sugar, and salt. Add cold butter and pulse until the mixture resembles coarse crumbs.

Add egg yolk and pulse; if too dry, gradually add cold water until it just holds together.

Turn out dough onto a lightly floured surface and knead briefly. Wrap and chill for 30 minutes.

Preheat oven to 350 °F (175 °C). Roll chilled dough to about ⅛-inch thickness. Cut circles to fit tartlet molds, press into molds, and prick bottoms with a fork.

Bake for 12–15 minutes until golden. Cool completely.

To make the curd: whisk lemon juice, zest, sugar, and eggs in a heatproof bowl. Set over simmering water (double boiler) and whisk constantly until thickened (~8–10 minutes). Remove from heat and whisk in butter until smooth. Strain curd through a sieve, cover with plastic wrap (touching surface), and chill at least 1 hour.

Spoon or pipe the chilled curd into the cooled tartlet shells. Optional: dust with powdered sugar or top with fresh berries or whipped cream. Chill before serving.