Preheat oven to 375°F. Cook pasta according to package directions, drain and set aside.
Melt 4 tbsp butter in a large pot over medium heat.
Whisk in flour to create a roux and cook for 1 minute.
Gradually add milk, whisking constantly until smooth.
Add cheddar and Gruyère cheese, stirring until melted.
Season with mustard powder, salt, and pepper.
Stir in cooked pasta and bacon.
Pour into a greased baking dish.
Mix panko breadcrumbs with 2 tbsp melted butter and sprinkle over top.
Bake for 25-30 minutes until golden and bubbly.
Garnish with fresh parsley and serve.