Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
Use a serrated knife to cube 12 English muffins into 1/2-inch pieces and add them to the baking dish. In a small bowl, melt 4 tablespoons salted butter in the microwave in 15-second increments. Stir in 1 teaspoon garlic powder. Drizzle over the cubed English muffins, tossing gently to coat. Bake for 10 minutes, or until the cubes are lightly browned.
While the muffin cubes bake, crack 18 large eggs into a large mixing bowl. Add 1 cup heavy cream and season with 1 teaspoon salt and 1 teaspoon black pepper. Whisk until fully combined. Stir in 2 cups shredded cheddar cheese.
Remove the toasted muffin cubes from the oven and sprinkle 1 pound diced ham evenly on top. Pour the egg mixture over the muffin cubes and ham, ensuring everything is well coated.
Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and continue baking for an additional 10 minutes, or until the eggs are set and the cheese is bubbly.
Allow the casserole to cool for 5 minutes before slicing. Serve with a drizzle of Hollandaise sauce.
Notes
Make Ahead Instructions: Prepare the casserole as directed, but do not bake. Cover and refrigerate for up to 12 hours. When ready to bake, follow the baking instructions, adding a few extra minutes if needed to ensure the casserole is heated through.