Remove from the heat and stir in 200g of the cheddar and 25g of the parmesan. (If you get any lumps, give it a vigorous whisk to remove them.) Heat, stirring all the time and scraping the bottom of the pan so it doesn't catch, until the sauce is bubbling and nicely thickened. Add the rest of the milk, a little at a time at first, then gradually add more as the mixture gets saucy. Splash in some milk gradually and stir until smooth and completely incorporated. Heat, stirring, until the butter has melted, then cook to a paste for 1 min. Wipe out the pan, then add the butter, flour and mustard powder. Drain the milk and discard the leek and bay.Take off the heat and set aside to infuse for 1 hr. Put the bay leaves, leek and milk in a pan, and gently heat until steaming but just below boiling point.Bake until the dish bubbles around the edges, about 15 minutes. Transfer the macaroni and cheese to the prepared baking dish and top with the buttered breadcrumbs.For the topping: In a small bowl, stir the breadcrumbs with the butter until the crumbs are moistened.Add the drained macaroni to the cheese sauce and mix thoroughly. Continue to cook, stirring constantly, until the sauce thickens. Slowly whisk in 1 cup of the milk and then whisk in the remaining 1 cup milk. Using a wire whisk, stir in the flour and salt, stirring and cooking over medium heat until the roux bubbles and the flour turns pale brown, about 3 minutes. For the cheese sauce: Meanwhile, melt the butter in a 1-quart saucepan.Bring the water back to a boil and cook the macaroni until tender, about 12 minutes.
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Bring 4 quarts of water to a boil in a large saucepan.For the macaroni: Preheat the oven to 350 degrees F.