Preheat oven to 350° F. Lightly grease an oven safe baking dish and set aside. I use a cast iron 12-in round skillet when I make this dish.
Fill a pot with about 2 inches of water and bring to a simmer. Add broccoli, cover, and steam for 2-3 minutes. Remove from heat and drain water. Place broccoli in the prepared baking dish and set aside.
In a medium pot, melt 2 tablespoons of butter. Whisk in flour and cook for about a minutes. Whisk in the milk, garlic powder, pepper, and salt. Bring the heat until the mixture is about to simmer and whisk in the shredded cheese. Whisk till melted and then whisk in the sour cream.
Pour the cheese sauce over the broccoli. In a gallon sealable bag, crush the crackers and add the melted butter. Reseal and toss the crushed crackers in the butter for the topping.
Top the broccoli casserole with the crushed buttered crackers and bake uncovered for 20 minutes. Remove from the oven and serve.