Preheat your oven to 400 degrees F. Melt 1 tablespoon butter in a wide, deep cast iron (or other ovenproof) skillet.
Once melted, pour the butter into a small mixing bowl, then combine with the Panko breadcrumbs and the Parmesan cheese. Set aside.
Meanwhile, bring a large pot of water to a boil. Add the broccoli and cook until crisp-tender, about 3 minutes. Drain and set aside.
Melt the remaining 2 tablespoons of butter in the same skillet. Sprinkle the white whole wheat flour over the top, then whisk constantly for 1 minute, until the butter smells fragrant and the flour turns slightly golden.
Very slowly pour in the non-fat milk, whisking constantly, adding a little at a time so that you break up the clumps of flour as you go. Continue whisking until the sauce thickens, about 3 minutes, allowing the mixture to bubble.
Stir in the garlic powder, salt, mustard, black pepper, and cayenne (if using). Remove the skillet from the heat, then stir in the cheddar until smooth.
Add the reserved broccoli, stir to coat, then pat the broccoli into an even layer in the skillet.
Sprinkle the Panko mixture evenly over the top of the broccoli. Bake until hot and the topping is crisp and light golden brown, about 20-25 minutes.
Let stand 5 minutes, then serve.
