Cook the spaghetti in salted water until al dente. Strain the cooking water and add the butter, cream, lemon juice and zest, herbs and parmesan to the hot pasta and mix well. Mix the chili powder with the pasta if you like.
Sear the prawns in olive oil with a sprig of rosemary and thyme and the finely chopped garlic cloves on both sides. Season with salt and pepper. Arrange the pasta in deep plates and place the prawns on top.