Wash the zucchinis and slice them in half lengthwise. Using a spoon, gently scoop out some of the flesh to create boats, leaving about a 1/4-inch border.
In a medium bowl, combine the shredded chicken, basil pesto, halved cherry tomatoes, garlic powder, salt, and pepper. Mix well to combine.
Drizzle the zucchini halves with olive oil and lightly season with salt and pepper. Place them on a baking sheet, cut side up.
Fill each zucchini boat with the pesto chicken mixture evenly.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese over the top of the filled boats.
Bake in the preheated oven for about 25-30 minutes, or until the zucchini is tender and the cheese is bubbly and golden.
Remove from the oven and let them cool slightly before serving. Garnish with fresh basil leaves if desired.