Melt the butter for the crust. Let it cool slightly, then crumble the shortbread cookies and mix them together.
Grease the tartlet molds with oil. Pour the butter cookie mixture evenly into each one. Firmly press the dough into the mold's edges and at the base. Chill for 30 minutes.
Wash one of the lemons with hot water and grate the zest. Then halve both lemons and squeeze out their juice. Top up the lemon juice with water to 1½ cups . Boil it together with the sugar and butter in a saucepan.
Mix the cornstarch with 4 tablespoons of cold water and add it to the simmering lemon mixture. Let it simmer for 1 minute and then remove the saucepan from the heat. Pour the cream into a bowl and let it cool for a few minutes.
Whisk the egg yolks with 2 tablespoons of the lemon cream and stir them into the lemon curd. Finally, fold in the lemon zest and distribute the cream in the tartlets.
Chill the lemon curd tartlets for another 20 minutes.