01 - Preheat oven to 350°F. Grease a 12-cup mini tart pan or muffin tin.
02 - In a bowl, mix flour, powdered sugar, and salt. Add cold butter and rub in until mixture resembles breadcrumbs.
03 - Stir in egg yolk and 1 tablespoon cold water to form a dough, adding more water if needed. Knead lightly on floured surface until smooth. Wrap and chill 15 minutes.
04 - Roll dough to 1/8-inch thickness and cut into 12 circles. Press circles into tartlet molds, trimming excess.
05 - Prick bases with fork. Chill 10 minutes. Line shells with parchment and fill with baking beans. Bake 12 minutes, remove beans and parchment, bake 5–7 minutes more until golden. Cool completely.
06 - Whisk eggs, egg yolks, and sugar in heatproof bowl. Stir in lemon juice and zest.
07 - Set bowl over simmering water and cook, stirring constantly until thickened and coating the back of a spoon, about 8–10 minutes.
08 - Remove from heat, add butter cubes, and stir until smooth.
09 - Spoon or pipe lemon curd into cooled pastry shells. Chill at least 1 hour before serving.