Rub the flour, salt, and butter together in a bowl until the mixture resembles breadcrumbs.
Mix in cold water with a knife to form a dough and knead briefly on a floured surface until smooth.
Shape the dough into a rectangle, wrap in plastic, and refrigerate for 20 minutes.
Roll the dough into a 40x20cm rectangle.
Fold the pastry into thirds, turn 90 degrees, and repeat the rolling and folding process.
Wrap the pastry, chill for 20 minutes, and repeat the folding steps again.
Chill the finished pastry block for 30 minutes before final use.
Preheat the oven to 200°C.
Simmer pears, 25g sugar, and vanilla seeds in water for 15 minutes until tender, then drain and dry.
Caramelize the remaining sugar in a 24cm ovenproof pan, then stir in butter, cream, and rosemary.
Remove the rosemary sprigs and arrange the pears cut-side up in the caramel.
Roll the pastry to 5mm thickness and cut a circle slightly larger than the frying pan.
Lay the pastry over the pears and tuck the edges into the pan.
Bake for 20-25 minutes until golden, then cool for 10 minutes.
Carefully invert the tart onto a large serving plate.
Slice the tart and serve warm with a scoop of vanilla ice cream.