First, make the pastry. Rub the butter into the flour with your fingertips until it resembles coarse breadcrumbs. Then stir in the sugar.
Mix in the egg yolk and cold water (about 1 tsp) until it forms a soft dough. Add more water, a little at a time, if your dough is too dry. Wrap the dough in cling film/plastic wrap and chill in the fridge for at least 30 minutes.
Roll out the dough on a well-floured surface until it’s about 0.5cm thick and cut out rounds to line your tartlet tins/bun tray.
Bake the tartlets at 325F/160C for about 15-20 mins until they are golden brown. It does help if you can use baking beans/pie weights while you’re baking the cases, as it will stop the pastry from rising slightly. But it will taste the same if you don’t. Leave cool in the tin.
After the cases have cooled, put 1 tsp jam into each and then place halved marshmallows (or 1 tbsp marshmallow fluff) on top.
Place the butter, sugar, syrup and condensed milk in a saucepan over a low heat. Stirring frequently, bring to a simmer and let bubble for 2-3 minutes until thickened.
Quickly spoon sufficient caramel to cover each marshmallow. Set aside to cool and firm up before enjoying.