Preheat the oven to 250F and put the popcorn in a big bowl.
In a medium saucepan, combine the brown sugar, corn syrup and butter and bring to a boil over medium heat. Reduce the heat and boil without stirring, swirling the pan occasionally, for 4 minutes. Remove from heat and stir in the baking soda and Sriracha - it will foam up at first, but stir it well to get any lumps of baking soda out.
Quickly pour over the popcorn and stir (with a heatproof spatula or tongs) to coat well. Spread out onto a rimmed baking sheet and bake for 30 minutes, stirring once or twice. Cool and break apart.