In a large, heavy bottomed saute pan or pot, melt the butter with the olive oil over medium heat. Add the onion, garlic, and a pinch of salt, and cook until the onions are translucent, about 10 minutes, stirring occasionally.
Stir in the tomato paste and red pepper flakes. Cook while stirring, until the tomato paste has darkened in color and is slightly caramelized, about 5 minutes. Add the vodka and tomatoes and bring to a boil, then reduce heat and maintain a high simmer uncovered for 30 minutes.
After 30 minutes, add 1.25 cups of the reserved pasta water to the sauce and puree until smooth using an immersion blender. (Alternatively, allow the sauce to cool off the heat for 30 minutes and puree in a blender in batches.)
Combine the remaining pasta water with the cream and stir into the sauce to your desired creaminess. Reheat the sauce gently over low heat (low heat will keep the sauce silky and avoid curdling) and toss with the pasta. Enjoy!