Put a rack in middle of oven and preheat oven to 450 degrees F. Line a baking sheet with parchment.
Whisk together mustard, vinegar, salt, and 1/4 teaspoon pepper in a large bowl. Add chicken, tossing to coat well.
Pulse English muffins in a food processor until finely ground. Add cheese, butter and remaining 1/4 teaspoon pepper and pulse until well combined. Transfer to a shallow dish or pie plate.
Dredge chicken one piece at a time in crumbs, coating completely and pressing gently so crumbs adhere then transfer to baking sheet. Bake until golden brown, and cooked through, 15 to 20 minutes.
Recipe courtesy The Gourmet Cookbook, Conde Nast Publications, 2004