Warm the broth: Add the chicken broth to a medium pot and bring it to a gentle boil over medium heat.
Cook the pastina: Stir in the pastina and reduce to a lively simmer. Cook, stirring occasionally, until tender, about 5–7 minutes.
Keep an eye on it—it cooks quickly.
Make it silky: In a small bowl, whisk the egg with 2 tablespoons of Parmesan. Lower the heat so the soup is barely simmering. Slowly drizzle in the egg mixture while stirring the soup in small circles.
It will thicken lightly and create delicate ribbons.
Add richness: Stir in the butter and remaining Parmesan until melted and smooth. Taste and season with salt and pepper.
Brighten it up: Add lemon juice, if using, and stir. Fold in any optional add-ins like spinach or cooked chicken; warm through for 1–2 minutes.
Finish and serve: Ladle into bowls.
Top with more Parmesan, cracked pepper, and a sprinkle of parsley. Serve hot.