1. Preheat oven to 375°F. Spread cashews in a single layer on a large baking sheet. Bake until warmed through, about 10 minutes, shaking pan once or twice.
2. Warm a small skillet over medium-high heat. When hot, add cumin seeds and toast, shaking pan constantly, until seeds are slightly darkened and aromatic, about 1 minute. Transfer to a bowl to cool completely. Spread on a cutting board and chop finely.
3. Combine chopped cumin seeds, cayenne, brown sugar, salt and butter in a large bowl. Add warm cashews and stir vigorously, breaking up any clumps of spice mixture. Let cool and serve. (Can be made up to 3 days in advance. Keep tightly covered in a cool, dry spot. Stir well before serving.)