Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Steam or blanch the broccoli florets until tender-crisp, about 3-5 minutes. Drain well and set aside.
Step 3: In a large saucepan or Dutch oven, melt 1/4 cup (1/2 stick) of butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 4: Gradually whisk in the milk until smooth. Bring the mixture to a simmer, stirring frequently, until it thickens, about 5-7 minutes.
Step 5: Reduce heat to low. Stir in 3 cups of the shredded cheddar cheese, cream cheese, garlic powder, onion powder, salt, and pepper. Continue stirring until the cheeses are melted and the sauce is smooth. Remove from heat.
Step 6: Gently fold the cooked broccoli florets into the cheese sauce.
Step 7: Pour the broccoli and cheese mixture into the prepared baking dish.
Step 8: In a small bowl, melt the remaining 1/4 cup (1/2 stick) of butter. Add the crushed Ritz crackers and toss until evenly coated.
Step 9: Sprinkle the buttered Ritz cracker mixture over the top of the casserole. Evenly spread the remaining 1 cup of shredded cheddar cheese over the cracker topping.
Step 10: Bake for 20-25 minutes, or until the casserole is bubbly and the Ritz cracker topping is golden brown and crispy.
Step 11: Remove from oven and let stand for 5-10 minutes before serving. This allows the casserole to set slightly.