This recipe is a love letter to fall in NYC and all the weekends I beg my handsome husband to go apple picking with me. Apple picking is just an excuse for a road trip with unlimited amounts of apple cider and doughnuts.
Heat the sugar and corn syrup in a heavy-bottomed medium saucepan over medium-high heat. As soon as the sugar starts to melt, use a heatproof spatula to move it constantly around the pan — you want it all to melt and caramelize evenly. Cook and stir, cook and stir, until the caramel is a deep, dark amber, 3 to 5 minutes.
Meanwhile, bloom the gelatin.
Once the caramel has reached the target color, remove the saucepan from the heat. Very slowly and very carefully, pour in the heavy cream. The caramel will bubble up and steam — stand away until the steam dissipates. Use the spatula to vigorously stir the mixture together. If it is at all lumpy or if there are any clumps of hardened caramel floating around the cream, put the saucepan back over medium heat and heat the mixture, stirring constantly, until all the caramel has dissolved and the mixture is smooth. Remove the pan from the heat.
Stir the bloomed gelatin into the caramel. Once the gelatin has dissolved, stir in the butter, vanilla extract, and salt. Pour the caramel through a fine-mesh sieve to make sure there are no lumps of sugar or gelatin in it.
Let the caramel cool to room temperature, then let it cool covered in the fridge for at least 8 hours before using. It will keep in the refrigerator, in an airtight container, for up to 3 weeks.