Roll out the pastry on a lightly floured surface and use to line four deep-fluted tartlet tins (8 x 3cm). Place the tins on a tray and chill for 30 mins. Heat oven to 190C/170C fan/gas 5. Line each pastry case with baking parchment and fill with baking beans. Bake for 10-15 mins, until the sides are set. Remove the parchment and beans and cook for 5-10 mins. Leave to cool. Can be made up to 1 day ahead at this stage and be stored in an airtight container. Lower the oven to 150C/130C fan/gas 2.
Pour the lactose free cream into a heavy-based saucepan, scrape in the seeds from the vanilla pod, then throw in the 2 halves of the pod. Heat until small bubbles begin to form around the sides of the pan, then leave to infuse for 5 mins. Remove the pod. In a bowl, beat together the egg yolks, whole egg and sugar. Keep stirring, then pour in the cream, mixing until combined. Strain through a sieve into a jug. Pour the custard into the tart shells, then bake for 18-22 mins until almost set (they should be quite wobbly in the centre but will firm up on cooling). Leave to cool completely, then chill for 30 mins.
Meanwhile, make the caramel topping. Grease a lipped metal baking tray with a little oil. Tip the caster sugar into a frying pan with 3 tbsp water and heat gently until the sugar starts to melt. Don’t be tempted to stir the sugar, as this can encourage the caramel to crystalise. However, you can tilt the pan to move it around. Once the sugar is almost melted, turn up the heat and bubble the caramel to a deep golden-brown colour. Quickly tip onto the greased tray and leave to cool completely. Once cool, turn the tray upside down and tap to release the caramel. Break into pieces, then whizz in a food processor until you have fine crystals.
Heat the grill. Scatter a thick layer of caramel crystals over the surface of each tart, and pop a collar of foil around the top edge of the pastry. Place under the grill, not too near the heat source, and let the caramel melt – watch carefully as they will burn easily. Leave to cool, then chill for 30 mins or until ready to serve.