Preheat oven to 450
Cook the bacon in a large skillet over medium heat (medium heat will render more fat and crisp the bacon more than high heat). Once the bacon is crisp, remove from pan with a slotted spoon, set aside.
Place the pan back on the burner and turn heat to high. Add the sprouts, cut side down, cooking until browned but not cooked through. Add sprouts to a baking dish, sprinkle with chopped bacon.
In a blender add the parmesan, mozzarella, cornstarch, beer, pepper, salt and mustard powder, blend until smooth.
Pour the cheese mixture over the sprouts.
Toss the bread crumbs with melted butter, spread evenly over the top of the cheese.
Bake, uncovered, until breadcrumbs have browned, about 15 minutes.