Combine the brown sugar and sweet paprika in a medium bowl, breaking up any lumps with your fingers until evenly sandy.
Add the kosher salt, chili powder, black pepper, garlic powder, onion powder, mustard powder, ground cumin, and cayenne pepper. Whisk until the color is uniform and the blend is fully combined.
Use immediately: Pat meat dry, then coat evenly with 1 to 1.5 tablespoons rub per pound, pressing to adhere. Let sit 20–30 minutes at room temperature, or cover and refrigerate 2–12 hours before cooking.
For grilling or smoking, cook over indirect heat to prevent scorching from the sugar; add a light second dusting near the end if desired.
Transfer any leftover rub to an airtight jar, label, and store in a cool, dark place for up to 3 months.