In a medium bowl, whisk together the light brown sugar, paprika, kosher salt, black pepper, chili powder, garlic powder, onion powder, cayenne pepper, mustard powder, and ground cumin until evenly combined and free of lumps, 1–2 minutes.
Spread the mixture on a sheet of parchment, break up any remaining clumps with your fingers, then funnel it back into the bowl.
Use immediately or transfer the rub to a clean, airtight jar.
To apply, pat meat dry, lightly oil if desired, then sprinkle 1–2 tbsp rub per pound, coating all sides; press to adhere.
Let the seasoned meat rest 15–30 minutes at room temperature (or refrigerate up to overnight) before cooking so the spices hydrate and cling.
Store leftover rub in a cool, dark place up to 6 months; shake or stir before each use.