Bring a large pot of salted water to boil
While the water heats, melt the butter and the chile crisp in a large skillet or Dutch oven over low heat. Whisk in the cream and keep warm over low. It should steam, not bubble
Cook the fettuccine until al dente. Use tongs to transfer the noodles to the cream mixture, reserving the pasta water. Add the spinach and turn with tongs until noodles are well-coated
Add parmesan and toss, still over low heat, until noodles are slicked with creamy sauce. Add pasta water a spoonful at a time as needed to loosen the sauce
Divide among dishes and serve with extra parmesan and chile crisp