Preheat the oven to 200°C/180°C fan/395°F. Start by making the caramel. Melt the butter over medium heat in your oven-proof pan. Once melted, sprinkle over the sugar. Combine everything with a wooden spoon, and stir regularly as the sugar melts. Keep your eye on it as the sugar will darken, you don't want it to burn. Remove from the heat when it is a smooth deep chestnut colour, about 5-10 minutes.If the butter and sugar don't meld and the sugar remains hard for 5-10 minutes, add 1 tbsp of hot water. Stir to combine and you should have a glossy caramel. Add the vanilla, cinnamon and salt to the caramel, stir to combine and allow to bubble for a few seconds depending if you want your caramel to darken any further. Remove from the heat so it doesn't continue to cook and burn.
Place the apricot halves either way up in the pan of caramel, lower the temperature and return the pan to the heat. Gently cook until the fruit is really soft, about 10 minutes. Make sure the heat is low as you don't want to apricots to burn.
Meanwhile, roll out the pastry into a rough circle, roughly the height of a £1 coin. Remove the pan of apricots from the heat. Cool for a couple of minutes so the heat doesn't immediately melt the pastry.
Working quickly, flip the pastry on top of the fruit and pan. Trim off the excess - you want it to be just the size of the pan. Tuck the pastry edges around the fruit.
Place the pan in the hot oven and bake for 30-40 minutes until the pastry is golden and crisp. Carefully remove the pan and leave to cool slightly before sliding a spatula around the edge to loosen the tart.
Now, the moment of truth - the flipping! Take your serving plate and place it over the pan. Holding a tea towel in both hands, grab the edges of the plate and pan and quickly flip it over. Put the plate on the work surface, tap the bottom of the pan and lift it away. The tart should be sitting there! Scrape all the extra caramel out of the pan on top of the tarte tatin. Serve with ice cream.