Warm the broth: In a medium pot, bring the chicken broth to a gentle boil over medium heat.
Cook the pastina: Stir in the pastina. Reduce to a simmer and cook according to package directions, usually 5–8 minutes, until just tender.
Add butter and cheese: Turn heat to low.
Stir in the butter until melted, then add the grated Parmesan. Taste and season with salt and black pepper.
Optional egg finish: In a small bowl, whisk the egg. Ladle a little hot broth into the egg while whisking to temper.
Slowly pour the egg mixture into the pot, stirring constantly, until the soup looks silky and slightly thickened, 30–60 seconds.
Adjust and serve: Add a squeeze of lemon if you like brightness. Ladle into bowls and top with more cheese and a sprinkle of parsley or red pepper flakes.