Heat a tablespoon of oil in a heavy bottomed sauce pan with a tight fitting lid.
Pour in some popping corn, how much all depends on how big your saucepan is. You don't need many kernels to get a pan full so err on the side of caution. You can always cook a second batch if it's not enough.
Whilst the corn is popping, mix the sugar, salt and chilli together in a small bowl.
Keep shaking the pan from side to side (lid firmly still on!) until the popping noises become less frequent.
Take off the heat, add the knob of butter and mix round quickly to coat as much popcorn as possible. Next, sprinkle the sweet chilli mix over generously and it'll stick to the melted butter and popcorn. Let cool before storing in an air tight tub.