Set a medium saucepan over high heat and add the oil and 2 popcorn kernels. Cover the pot and wait until both kernels have popped. Add the remaining kernels, cover, and shake the pot continuously until all have popped and you hear a 2-3 second gap between popping sounds. Transfer the popped popcorn to a bowl and immediately season generously with salt. Set aside.
Preheat an oven to 100°C/210°F. Line a large baking tray with parchment paper and set aside.
Add sugar to a small saucepan and pour over 90ml (⅓ cup) of cool water. Set over low heat and, stirring constantly, allow the sugar to completely dissolve.
Wash down the sides of the pot with a pastry brush dipped in water to prevent any sugar from crystallising. Once the mixture has come to a boil, stop stirring and allow to boil until the sugar mixture reaches a deep amber colour, whisps of smoke appear and an instant-read thermometer registers at 160°C/320°F. DO NOT stir the mixture at all once it has come to a boil.
Remove the pot from the heat and stir in the butter. It will sputter and foam, so do not be alarmed. Add the salt, baking soda and vanilla and stir until thoroughly combined.
Pour the caramel over the popped popcorn, stirring until all of the popcorn is completely coated. Pour the popcorn into the prepared baking sheet, spreading it out to ensure it is in an even layer.
Transfer the popcorn to the oven and cook for about 1 hour, or until the popcorn has dried out and the caramel has set. Allow to cool for about 15 minutes before breaking up and transferring to an airtight container - or enjoying right away!
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Notes
I recommend using caster sugar (also known as superfine sugar) here as it dissolves more easily and quickly in the water. However, you can use granulated sugar if this is all you have.