Boil a large pot of salted water. Add pasta of choice and cook to al dente. Drain, reserving 1 cup pasta water.
Heat olive oil in a medium skillet over medium heat. Add finely chopped onion, garlic, red pepper flakes, salt and pepper. Sauté 3-5 minutes, stirring constantly, until onions are tender.
Add the tomato paste and cook over medium heat, stirring constantly, 2-3 minutes or until the color turns bright red.
Add the vodka and cook, stirring, until vodka is incorporated and sauce is thickened.
Slowly whisk in the cream and reduce heat to low. Add the pasta water and whisk to incorporate.
Add in the grated Parmesan and butter, stirring vigorously to combine.
Add additional pasta water as needed until desired consistency is reached.