Bring 6 quarts generously salted water to a boil.
While the water heats, melt the butter in a large, deep skillet over medium-high heat. Add olive oil, garlic; sauté until fragrant and sizzling, about 2 minutes. Whisk the cream, egg yolk and ricotta in a bowl until blended; pour into the garlic butter. Reduce heat to medium-low; stir until hot but not boiling. Keep warm over low heat.
Meanwhile, cook the pasta, partially covered, until al dente. Reserve about a cup of pasta water before draining in a colander. Pour hot pasta into the cream mixture and toss to coat (still over low heat). Add the cheese and fresh parsley and keep tossing gently until cream is mostly absorbed. Season generously with salt and pepper. If pasta is not as creamy as you would like, toss with a little of the reserved pasta water. Serve in warm bowls.
