Preheat the oven to 180C/250F/Gas 4.
For the caramelised butternut squash, place the sugar into a small ovenproof frying pan and heat gently until melted and caramel-coloured. Add the butter and stir to combine.
Add the butternut squash pieces, stir well and cook gently for 3-4 minutes, until softened.
For the pastry, place the crushed pine nuts, plain flour and cold butter into a food processor and pulse until the mixture resembles breadcrumbs.
Add the beaten egg and a splash of milk if the mixture is still too dry and blend to combine.
Turn the dough out onto a floured surface and roll out to the thickness of a pound coin.
Place the pastry over the cooked butternut squash in the pan, pressing down around the edges to enclose the filling and trimming off any excess pastry.
Transfer to the oven and bake for 12-14 minutes, or until golden-brown.
Remove the tart from the oven and allow to cool slightly. Slide a knife around the edges of the frying pan and turn the tart out onto a serving plate.
To serve, mix the yoghurt and chives together in a bowl and place a dollop of chive yoghurt on top of the tart.