Preheat a grill to medium. Whisk the yellow mustard, vinegar, brown sugar, mustard powder, hot sauce, Worcestershire sauce, 1/2 teaspoon salt, and pepper to taste in a bowl. Whisk in the butter. Season the chicken with salt and pepper, then toss with about one-third of the mustard sauce in a large bowl until coated. Let sit at room temperature, 10 minutes. Brush the grill grates generously with vegetable oil. Put about 1/4 cup of the remaining mustard sauce in a small bowl for basting and reserve the rest for topping. Grill the chicken, covered, basting occasionally with the sauce, until well marked and a thermometer inserted into the thickest part registers 170 degrees F, 10 to 12 minutes per side. Serve the chicken with the reserved mustard sauce, sliced tomato and macaroni salad, if desired.