To make the shortbread shells, preheat the oven to 350?F. In a bowl (or the bowl of a food processor), combine the flour, sugar and salt; add the butter and blend (or blitz) until well combined and crumbly. Press the crumbs into the bottom and up the sides of mini muffin tins and bake for 8 minutes, or until pale golden.
In a medium saucepan, whisk together the egg yolks, sugar, lemon juice and zest. Set over medium heat and cook, stirring often (if not constantly) with a whisk, until the mixture comes to a boil and thickens. Remove from the heat and stir in the butter. Set aside to cool slightly, then spoon into shells.
Makes about 3 dozen tarts.