Cook the pasta according to package directions to al dente.
Meanwhile, in a large skillet, sauté the garlic and onions in 2 tablespoons of the butter and the olive oil over medium-low heat until soft, but not browned (if you burn the garlic, start over!).
Carefully pour in the tomato puree, and then the vodka. Stir the mixture until it's thoroughly combined, and cook for about 2 minutes—the alcohol in the vodka will burn off.
Reduce the heat to low. Pour in the heavy cream. Stir to combine, then turn heat to the lowest possible simmer while you wait for the pasta to be done.
Sprinkle in the red pepper flakes and add salt and pepper to taste. Finally, stir in the remaining 1 tablespoon butter.
Drain the pasta (do not rinse!) and add to the sauce; toss to combine. To serve, add a serving of the pasta to a pasta bowl or plate, then sprinkle on some parmesan, parsley, and black pepper. Serves 4-6.