• Line a large, rimmed baking tray with foil and grease well with oil. Preheat the oven to 150°C.

Make the popcorn:

• Place the 1 tbsp of oil into a large, heavy-bottomed saucepan over medium-high heat. Add 3 popcorn kernels, then cover with a lid and wait, shaking gently every now and then, until all three of the kernels have popped. At this point, quickly add the rest of the kernels, replace the lid and shake frequently as the popcorn begins to pop. When the popping slows to several seconds between pops, remove the pan from the heat. Tip the popcorn onto the baking tray and spread out into an even layer. Once cool, pick out and discard any un-popped kernels, then set the tray aside while you make the toffee.

Make the toffee:

• Place the butter, brown sugar and golden syrup into a large saucepan over medium heat. Stir frequently until the butter has melted, then increase the heat and bring the mixture up to a boil. Turn the heat down to low and allow the toffee to simmer undisturbed for 10 minutes, until it is a deep golden colour.
• When the toffee has 3 minutes left of cooking time, place the baking tray, with the popcorn still on it, into the oven to warm up. This will make tossing the popcorn and the toffee together easier.
• When the toffee is ready, remove the pan from the heat and add the flaky sea salt and vanilla, stirring to incorporate. Be careful, because the mixture will bubble up a little- keep stirring until the bubbles subside and a smooth mixture forms. Remove the tray of popcorn from the oven and pour the toffee over the top. Toss quickly with two spatulas to coat the popcorn.
• Place the popcorn into the preheated oven and bake for 20 minutes, stirring every 5 minutes to ensure the popcorn is evenly coated. Remove from the oven and allow to cool slightly before breaking up the popcorn with your hands (this is easiest when the popcorn is still ever so slightly warm). Stored in an airtight container at room temperature, the toffee popcorn will keep for about a week.