Prepare pasta according to package directions, using a very large pot. Add the broccoli for the last 3 minutes of cooking. Reserve 1/2 cup of the pasta cooking water.
Drain the pasta and broccoli in a colander. Return to the pot. Toss with the butter.
Meanwhile, add the ricotta, lemon zest, lemon juice, salt, Parmesan cheese, and 1/4 cup cold water to blender. Blend smooth.
Add the cheese mixture to the pot with the pasta and broccoli. Use a spatula to toss the pasta until coated, adding a little of the reserved pasta water at a time until a smooth, but thick sauce forms.
Transfer pasta to a large bowl and top with additional cheese to serve—if desired.
Store any leftovers in an airtight container for up to 5 days. Reheat to serve, stirring in a little of the reserved pasta water, regular water, or broth if needed.