Preheat the oven to 350°F/18o°C/gas 4. Position a rack in the center.

Fill a 12-in ovenproof skillet, preferably cast iron, with water up to about 1 in/2.5 cm from the top. Cover and bring to a boil over high heat. Add 2 tsp salt and toss in the macaroni. Stir gently once or twice so the pasta doesn't stick.

Reduce the heat to medium-high. You must cook the macaroni at a gentle boil only until it's still just short of completely tender, because it will finish cooking in the oven. For example, if the box says to cook for 7 minutes, test the pasta after 5 minutes.

To check, fish a piece out of the water (a slotted spoon makes it easy), run it under cold water, and bite it. It should still be chewy, but not tough. Drain the pasta in a colander set in the sink and run cold water over it to stop the cooking and keep it from clumping.

Add 2 tbsp of the butter to the pan and melt over medium-high heat. Add the prosciutto and toss it around in the pan until it's crispy, about 2 minutes. Transfer the prosciutto to a plate. Put the panko in a small bowl. Pour the hot butter left in the pan over the panko and toss to coat.

Return the pan to medium-high heat and add the remaining 2 tbsp butter. When the butter is melted, sprinkle in the flour and 1/4 tsp salt. Cook, stirring, until the flour becomes foamy, about 1 minute. Whisk in the warm milk. Stir until the mixture is thickened and saucy, about 2 minutes. Remove from the heat and add all three cheeses, the nutmeg, and a grind or two of pepper, stirring until the cheeses are melted. Taste and season with more salt and pepper if it needs it. Stir in the macaroni and prosciutto until all of the mac is thoroughly coated and the prosciutto is evenly distributed. Smooth the top and sprinkle the buttery bread crumbs over the top.

Transfer to the oven and bake for about 20 minutes, or until the juices are bubbly and the top is lightly browned. If you have time to give it a few more minutes, the top will get even crispier. Scoop into warmed shallow bowls and serve hot.