Preheat the oven to 180°C.
Heat sugar in an ovenproof frying pan until it melts and turns caramel-coloured.
Stir in the 55g of butter until combined.
Add butternut squash and cook gently for 4 minutes until softened.
Pulse pine nuts, flour, and 85g of cold butter in a food processor until crumbly.
Blend in the egg and milk until a dough forms.
Roll the dough out on a floured surface to roughly 3mm thickness.
Drape pastry over the squash and press edges down to seal.
Bake for 14 minutes until the pastry is golden-brown.
Cool slightly, loosen edges with a knife, and invert onto a plate.
Mix yoghurt with chives and serve a dollop on the tart.