Boil pasta according to package directions for al dente. Salt the water well before cooking the pasta (should taste like the ocean). Heat butter over medium-low heat in a large, deep sauté pan. Add the onions and season with salt and pepper. Cook, stirring occasionally, until softened, about 5-7 minutes. Add the garlic, stir and cook until just fragrant, 1 minute. Add the tomato paste and cook stirring occasionally, until darkened in color, 5 minutes. Add the vodka and cook until evaporated, about 1 minute. Add the heavy cream and red pepper flakes. Cook, stirring well occasionally for 2 minutes, until slightly thickened. Add the drained cooked pasta and reserved pasta water, and toss to coat. Turn off heat, add parmesan and stir. Serve with additional grated parmesan and fresh basil leaves.