Preheat the oven to 375°F (190°C).
Flatten the chicken breast to about 1/2 inch thick, season with salt and pepper, and place on a baking sheet.
Bake the chicken at 375°F for 10 minutes, then remove and let cool until it can be handled.
Meanwhile, use a melon baller or spoon to scoop out the centers of the halved zucchini, leaving a thin wall around each boat; reserve the scooped zucchini flesh and roughly chop it into 1/2-inch pieces.
Roughly chop or shred the cooled chicken and add it to a large bowl with the chopped zucchini flesh, parmesan, 1/2 cup mozzarella, 1/3 cup barbeque sauce, garlic powder, and 2 teaspoons chopped cilantro; toss until evenly combined.
Spoon the mixture into each zucchini half, mounding slightly, and sprinkle additional mozzarella on top as desired.
Bake the filled zucchini boats at 375°F for 10–14 minutes, until the cheese is melted and the zucchini is tender.
Remove from the oven and let cool on a wire rack for 10 minutes, then garnish with extra chopped cilantro and serve.