Line a baking sheet with parchment paper and set it aside.
Melt the butter over medium high heat in a large frying pan. Add the brown sugar and stir to combine.
Continue cooking until the sugar melts, about 3 to 5 minutes, stirring every minute.
Add the nuts and cook until they are toasted and very fragrant, about 8 minutes, stirring every minute.
If the pan begins smoking, remove it from the heat and stir it until it stops smoking, then place it back over the heat and continue stirring every minute until the nuts are sufficiently toasted.
Empty the pan contents onto the parchment paper-lined baking sheet and spread the nuts mixture out evenly using a spatula. Immediately sprinkle with the rosemary, salt, and chili. Allow to cool to room temperature.
Once cooled, break apart any large chunks of nuts into individual nuts and serve.