Cook the caramel:
Bring the butter and sugar mixture to the boil, stirring constantly. Once boiling, stop stirring and cook for 3 to 4 minutes without stirring the mixture. It should reach a temperature of around 250°C (120°C) on a candy thermometer.
Add the baking soda and vanilla:
Remove the pan from the heat and add the baking soda and vanilla extract (if using). The mixture will bubble and rise a bit, so be careful.
Pour the caramel over the popcorn:
Quickly pour the hot caramel over the popcorn in the large bowl. Try to distribute the caramel evenly over the popcorn.
Mix and let cool:
Gently toss the popcorn with a spatula or wooden spoon to coat it evenly with the caramel.
Let cool and enjoy:
Let the popcorn cool and harden before eating.
Tip: If you want softer, less sticky popcorn, you can spread it on a tray lined with parchment paper so it cools and separates more easily.
Remember that caramel can be very hot, so be careful when handling it. Enjoy your homemade caramel popcorn!