Bring a large pot of lightly salted water to a boil. Add penne and cook according to package instructions until just al dente, about 9 minutes. Scoop out 1/2 cup pasta water and set aside. Drain pasta.
Meanwhile, heat olive oil in a large skillet over medium-low heat and cook onion until soft and translucent, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in tomato paste and cook, stirring, until tomato paste starts to darken and is well combined with the onion, 2 to 3 minutes.
Pour in heavy cream and vodka and cook, stirring, for 2 minutes until alcohol evaporates. Stir in red pepper flakes and cook, stirring, until well combined. Season with salt and pepper.
Stir in butter until melted. Add drained pasta and 1/4 cup pasta water. Add more pasta water depending on how thick you like the sauce. Stir in freshly grated Parmesan cheese, taste, and adjust seasoning with salt and pepper. Continue cooking for 1 to 2 minutes over low heat until well combined.
