Fill a large pot with water and salt it generously — it should taste like the sea. Bring it to a rolling boil over high heat.
Drop the pasta into the boiling water and cook according to package directions until just al dente. The shells should have a slight bite but no chalky center when you test one.
⏱ 10 min
Before draining, scoop out half a cup of the starchy pasta water and set it aside — this liquid gold will help create a silky sauce. Drain the pasta thoroughly.
Return the drained pasta to the pot and place it over low heat. This gentle warming helps the butter melt evenly without breaking.
Add the butter to the warm pasta and toss continuously until it melts completely and coats every shell with a glossy sheen.
⏱ 1 min
Fold in the diced ham and continue tossing until the pieces warm through and start to release their smoky aroma.
⏱ 2 min
Pull the pot off the heat immediately and add the grated Gruyère, tossing vigorously until it melts into a creamy coating. The residual heat will melt the cheese perfectly.
If the pasta looks dry or the cheese isn't melting smoothly, gradually add the reserved pasta water, one tablespoon at a time, until you achieve a silky consistency.
Taste and adjust with salt and several grinds of black pepper. Remember, the ham and cheese are already salty, so go easy at first.
Divide among warm bowls and scatter with fresh parsley if using. Serve immediately while the cheese is still melty and the pasta is piping hot.