Preheat the oven to 200°C and line four 8cm crumpet rings with greaseproof paper.
Melt the sugar in a heated pan then remove it from the heat.
Warm the lined tins in the oven for 5 minutes before pouring in the hot caramel.
Place two pineapple slices into each ring on top of the caramel.
Cut four pastry circles and tuck them over the pineapple, reaching the caramel base.
Place the rings on a lined baking tray and brush the pastry with beaten egg.
Bake for 20 minutes until the pastry turns golden-brown.
Whisk maple syrup, rum, lime juice, and 4 tablespoons of pineapple syrup together.
Invert the tarts, remove the paper, and serve with ice cream, syrup, and lime zest.