Heating the pan beforehand results in a nice crispy bottom.
Preheat oven to 425°. Brown ham in 1 Tbsp. melted butter in a nonstick skillet over medium-high heat 5 to 6 minutes.
Heat a 12-cup muffin pan in oven 5 minutes.
Combine cornmeal mix and sugar in a medium bowl; make a well in center of mixture.
Stir together buttermilk, egg, and Dijon mustard; add to cornmeal mixture, stirring just until dry ingredients are moistened. Stir in remaining 3 Tbsp. melted butter, cheese, and ham. Remove pan from oven, and coat with cooking spray. Spoon batter into hot muffin pan, filling almost completely full.
Bake at 425° for 15 to 20 minutes or until golden. Remove from pan to a wire rack, and let cool 10 minutes.
We tested with White Lily White Cornmeal Mix.