In a large pot, bring salted water to boil. Add the spaghetti and cook it al dente according to the package instructions.
While the pasta is cooking, grind the peppercorns with a mortar and pestle to a fine consistency.
Melt the butter over medium-high heat in a large skillet. When foamy, add the black pepper and toast until fragrant, 1- 2 minutes. Turn off the heat.
Add the Boursin cheese and 1/2 cup of the pasta water. Whisk the sauce until smooth.