Preheat your oven to 370°F (180℃) . Lightly grease a 9x13 inch baking dish with cooking spray.
Season chicken cubes with salt, pepper, and garlic powder.
Heat olive oil in a large skillet over medium-high heat. Add chicken and cook until golden and just cooked through, about 5-6 minutes. Remove from heat and set aside.
Bring a large pot of salted water to a boil. Add broccoli florets and blanch for 2 minutes until bright green but still firm. Drain immediately and rinse under cold water to stop cooking.
In a large mixing bowl, combine sour cream, 1½ cups of the shredded cheddar, and onion powder. Mix until smooth.
Fold in the cooked chicken and blanched broccoli, gently stirring to coat everything evenly.
Transfer the mixture into your prepared baking dish, spreading it out evenly.
Sprinkle the remaining ½ cup of cheddar cheese over the top.
In a small bowl, mix the melted butter and grated Parmesan. Pour this mixture evenly over the casserole.
Bake uncovered for 30-35 minutes, until edges are bubbling and topping is golden brown.
Let rest for 5 minutes before serving.