Make the caramel. Put the sugar and water in a medium heavy-bottomed pan. Bring to the boil and cook, swirling the pan, until you have a golden caramel – this will take several minutes.
Remove the caramel from the heat and pour in the cream – be careful, this will cause the caramel to bubble and rise in the pan. Return to the heat and simmer for 3-4 minutes
Add the butter and a a generous pinch of sea salt. Stir until the butter melts and is incorporated.
Pour the caramel into a jar and cool. Store in the fridge if you have leftovers – it will keep for a week.
Preheat the oven to 180C (350F). Line a baking tray with greaseproof paper and sprinkle the paper with a little sugar.
Put the block of marzipan in the freezer to chill for 10 minutes. Grate the marzipan using a box grater and set aside. You only need 3-4 tbsp of grated marzipan per tart.
Slice the apples thinly either with a sharp knife or a mandoline. You can slice into wedges or cut the whole apple into thin disks.
Take the puff pastry out of the fridge and let it stand for 10 minutes. Unroll carefully and use a plate to trace two circles of puff pastry - about 15cm /6in each.
Trace a smaller circle inside each tart to create a thin border and score inside it with a fork - do not pierce all the way through. You can cut small decorative shapes with the leftover pastry if you like and use them to top the tart.
Transfer the tarts to the prepared tin (you can use the paper the puff pastry came rolled in).
Brush with a little melted butter. Sprinkle the grated marzipan inside the border of each tart.
Arrange the apple slices in a fan shape over the marzipan and brush with butter. Sprinkle with a little sugar.
Bake for 30 minutes, rotating the tray halfway through. Take out of the oven and leave to cool slightly.
Serve drizzled with salted caramel and a scoop of vanilla ice cream if you like.