- Preheat the oven to 180°C / 350°F / gas mark 4 and well butter 2 baking trays.
- In a large mixing bowl, cream together the butter and sugar and beat until pale and fluffy.
- Add the vanilla extract, egg and egg yolks, one at a time, beating well between each addition.
- In another bowl, mix together the lemon rind, cornstarch, flour, baking powder and salt then add to the egg mixture and beat well to thoroughly combine.
- Drop small spoonfuls of the mixture onto the prepared baking trays, spacing well apart, then bake for 15 minutes.
- Cool on wire trays then sandwich the biscuits together with (dulce de leche) jam milk.