Roll out the pastry on a floured surface to 5 mm (¼ inch) thick. Cut in half and sprinkle one half with the vanilla sugar. Lay the second piece on top and thinly roll out the pastry. Cut out 12 rounds using a 9 cm (3¾ inch) plain biscuit cutter. Re-roll the trimmings to make more.
Press the rounds into the holes of a 12-hole nonstick muffin tray, pressing firmly into the sections. Line the pastry cases with squares of foil. (To do this, wrap each foil square tightly around half a lemon, then remove. Press the foil domes firmly into the pastry cases.)
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Remove the foil and bake the cases for a further 5 minutes until crisp. Reduce the oven temperature to 160°C (325°F), Gas Mark 3.
Meanwhile, beat together the eggs, egg yolks, caster sugar and vanilla in a heatproof bowl. Bring the cream to the boil in a saucepan and pour over the egg mixture, whisking well. Strain into a jug and pour into the cases. Bake in the oven for 15–20 minutes or until just set and still slightly wobbly in the centre. Leave to cool in the tin. Serve dusted with sifted icing sugar.