Put the sugar in a small saucepan with 2 tbsp water and dissolve over a gentle heat. Bring to the boil, but don’t stir at all. When the sugar is a dark golden caramel colour, whisk in the butter. Remove from the heat and pour in the double cream (stand back as it may spit), stir vigorously, then add the ground cinnamon.
Pop the popcorn kernels in a saucepan with a tight-fitting lid for 4 mins, then remove from the heat and pour over the sugar glaze, tossing the popcorn until evenly coated.