In a separate small bowl, whisk together the brown sugar, smoked paprika, salt, pepper, mustard powder, and cayenne. This is where the magic starts. The brown sugar should break up any lumps so the mixture is uniform and fine. Take a moment to smell this combination—that's what you're about to taste in about 15 minutes.
Sprinkle this entire mixture over your oiled chicken pieces in the large bowl. Now, here's the technique: toss everything together using your hands, making sure every single piece gets evenly coated. I spend about two minutes doing this, turning each leg and thigh to ensure the rub covers the skin completely, even the bottoms and crevices.