Cook pasta to al dente according to package instructions. Reserve 1 cup of the cooking liquid before draining (you may not need all of this, but I always reserve 1 cup).
While pasta is cooking, over medium heat, add olive oil to a large pan. When oil is hot, add onion. Cook for 2-3 minutes, then add garlic and cook 1 minute more. Stir often so garlic doesn't burn.
Add tomato paste and cook for 2-3 minutes, stirring often. Tomato paste will turn a slightly darker color.
Add vodka (if using) and cook for 1 minute. To make this dish without vodka simply omit this step OR use one of the substitutions listed in Note 1.
Add heavy cream and red pepper flakes. Cook for 1-2 minutes, stirring often until sauce thickens (it will thicken up quickly!).
Add in your pasta, ½ cup of pasta water and butter. Stir, stir, stir until butter is "melted and saucy" according to Gigi! :) Make sure the pasta is completely coated in the sauce. Add a little more pasta water if sauce seems too thick.
Stir in Parmesan cheese. Make sure you TASTE and season with salt and pepper if needed. Sprinkle with basil and serve immediately.
