Cook your pasta in a pot of well salted water (this sauce only takes a few minutes to bring together so judge the timing of cooking your pasta based on whether you are using fresh or dried).
Melt the butter over medium heat
When it is just starting to sizzle add the nduja and whisk into the butter, lower the heat if it's spitting and crackling too much.
Add the cream and whisk well to combine. You want a gentle simmer this whole time, it shouldn't be boiling at any point.
Add the lemon juice and sugar and stir to combine
Add the parmesan in a few lots, whisking well in between to make sure it has melted into the sauce.
Check for seasoning, it shouldn't need salt but a few cracks of black pepper will be perfect!
Add your cooked pasta straight into the sauce and gently stir.
Add the parsley and stir.
I like to let the pasta gently bubble away in the sauce for a minute or two to thicken up.
Serve with extra parmesan and a good crack of black pepper.