Preheat your oven to 375°F. Rinse the mushrooms gently under cold water and pat them dry. Remove the stems from each mushroom cap.
In a mixing bowl, combine the shredded chicken, basil pesto, softened cream cheese, and shredded mozzarella cheese. Add garlic powder, salt, and black pepper. Mix until well combined.
Stuff each mushroom cap with the chicken mixture, filling them generously but avoiding overstuffing.
Place the stuffed mushrooms on a baking sheet lined with parchment paper. Sprinkle with grated Parmesan cheese. Bake for 20-25 minutes until the mushrooms are tender and the cheese is bubbly and golden.
Let the stuffed mushrooms cool for a few minutes before serving. Garnish with fresh basil.