Melt 2 tablespoons of butter in a deep skillet over medium heat. Add garlic and sauté until fragrant, about 1 minute.
Add 6 tablespoons butter to the skillet and stir constantly until melted. Stir in half-and-half, chicken stock 1/2 teaspoon salt, 1/4 teaspoon garlic powder, black pepper, and cayenne. Add tortellini to the skillet and stir. Bring to a boil, then reduce heat to medium-low. Cook, stirring occasionally, until tortellini are tender and sauce is bubbly and slightly thickened, 6 to 10 minutes.
Gradually stir in 3/4 cup Parmesan cheese until melted and sauce has thickened. Stir in chicken. Remove from heat and sprinkle mozzarella cheese over the top.
Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.
In a microwave-safe bowl, melt the remaining 2 tablespoons butter in the microwave. Stir in breadcrumbs, remaining 1/4 cup Parmesan cheese, remaining 1/2 teaspoon garlic powder, remaining 1/4 teaspoon salt, and Italian seasoning. Spread evenly over the top of the tortellini.
Place the skillet under the broiler and broil until mozzarella is melted and breadcrumbs are toasted.
