In a medium bowl, stir together the chicken, pesto, and melted butter until chicken is evenly coated.
Place English muffin halves, cut sides up, on a baking sheet. Divide the chicken mixture evenly among the muffin halves. Bake about 10 minutes or until chicken mixture is heated through.
Top muffins with the bacon pieces; sprinkle with mozzarella cheese. Bake for 2 to 3 minutes more or until the cheese is melted and bubbly.