Boil pasta as per package instructions and set aside with reserved pasta water
While pasta is boiling, in another pan, add olive oil, butter and onions on medium low heat and cook down until onions have softened
Add salt, pepper, and chili flakes and let the spices cook down with the onions for a few minutes
Then add the tomato paste and minced garlic and mix thoroughly until the tomato paste is cooked down properly (5-7 minutes) and the raw smell of garlic is gone
Add the vodka, Calabrian chili sauce, and heavy cream and mix together and let simmer for a few minutes
The sauce will thicken up a bit and that is when you add the cooked pasta and reserved pasta water
Mix until fully combined and garnish with Parmesan cheese and serve hot!