Preheat the oven to 350°F. Grease a 9x13-inch baking dish with cooking spray. (if your mushrooms are on the larger size, arrange them on a baking sheet or appropriate size dish.)
Remove the stems from the mushrooms and set them aside. Using a damp paper towel or kitchen towel, wipe off the tops of the mushrooms to remove any dirt. Arrange the mushrooms, bottoms up, in the baking dish.
Finely chop the mushroom stems.
Add the Italian sausage to a medium skillet set over medium heat. Cook, breaking apart the sausage with a spatula, until it is no longer pink. Add the chopped mushroom stems and garlic to the skillet and continue cooking for 2 minutes.
Transfer the mixture to a medium bowl and let it cool slightly.
Add the cream cheese, Parmesan cheese, 2 teaspoons of the chopped chives and 1/4 teaspoon black pepper to the bowl. Stir to combine.
Divide the sausage mixture evenly among the mushrooms.
In a small bowl, stir together the Panko breadcrumbs, melted butter and remaining 1 teaspoon chives. Divide the breadcrumb mixture evenly among the tops of the mushrooms.
Bake the mushrooms until they are tender and the tops are golden, about 20 minutes. (If desired, place under the broiler for an additional 2 to 3 minutes to brown the tops.) Remove the mushrooms from the oven and serve.