Heat oven to 220C/200C fan/gas 8. Cook the macaroni in salted water for 2 mins less than the stated cooking time, so it still has plenty of bite. Drain well and toss with a small knob of the butter to stop it from sticking together.
In a heavy-bottomed saucepan, heat the remaining butter until frothy and nut-brown. Pour off 1 tbsp into a small bowl and place the pan back on the heat, then scatter over and whisk in the flour and cook for 3 mins until you have a sandy paste. Add the milk, a splash at a time, simmering and whisking until smooth between each addition. Once all the milk has been added, drizzle in the evaporated milk, then add the grated cheddar, a handful at a time, until you have a thick, cheesy sauce. Stir through the mustard powder, cayenne, yeast extract and vinegar and taste for seasoning, adding salt and pepper as needed.
Remove from heat, stir in the macaroni, then stir through the chunks of cheese. Tip the contents of the saucepan into a large, buttered baking dish. In a small bowl, mix together all the breadcrumbs, the parmesan, another small pinch of cayenne and some crunchy sea salt if you want. Scatter the crumbs over the macaroni, drizzle with the reserved brown butter and bake for 30-35 mins until lightly browned, crunchy all over and bubbling around the edges. Leave to stand for 10 mins before serving straight from the dish.
