Pre-heat the oven to 350 degrees F.
Pre-roast cashews: spread the cashews onto a parchment-lined baking sheet in an even layer and roast them for about 15 minutes, until they show a little color. Then transfer the nuts to a large mixing bowl, reserving the paper.
Finely chop the rosemary if you haven’t already.
Melt the butter in a small saucepan. Add in the hot water and brown sugar. Bring the mixture to a low simmer to combine them, careful not to over stir.
Turn off the heat but leave the pan on the burner and mix in the following: chili powder, salt (unless the cashews you are using are salted), and freshly chopped rosemary.
Add the sauce to the mixing bowl with the toasted cashews and quickly toss them until they are evenly coated.
Transfer the coated cashews back to the baking sheet with parchment paper and bake them for another 6 minutes.
Remove them from the oven and let cool for 20 minutes and seal them in an airtight container.
They will last about 3 weeks stored this way.