Bring a large pot of generously salted water to a rolling boil. Cook 1 lb of penne according to package directions until al dente. Reserve 1 cup of starchy pasta water before draining.
In a large deep skillet over medium heat, warm olive oil and melt butter. Add finely diced onion with a pinch of salt and cook 6-8 minutes until soft and just golden at the edges.
Add minced garlic and crushed red pepper flakes. Cook 1-2 minutes until fragrant and lightly golden, being careful not to burn.
Push aromatics to the sides and add tomato paste to the center of the pan. Cook 2-3 minutes, stirring frequently, until it darkens to a rich brick-red color.
Pour in vodka and deglaze the pan, scraping up browned bits. Simmer 2-3 minutes until reduced by half and the alcohol smell dissipates.
Stir in the crushed San Marzano tomatoes and season with salt and pepper. Bring to a gentle simmer and cook uncovered 15 minutes, stirring occasionally, until slightly thickened.
Using an immersion blender, blend the sauce directly in the pan until completely smooth and velvety. Skip this step for a chunkier sauce.
Return sauce to medium-low heat. Stir in heavy cream until it turns a soft blush-pink. Simmer gently 3-5 minutes, stirring frequently, without boiling.
Remove from heat and stir in grated Parmesan until fully melted and incorporated. Adjust seasoning with salt and pepper to taste.
Add drained penne to the sauce and toss over low heat 1-2 minutes. Add reserved pasta water a splash at a time if needed to reach desired consistency.
Serve immediately in warm bowls topped with additional Parmesan and torn fresh basil leaves.