Bring the chicken broth to a gentle boil in a medium saucepan over medium-high heat.
Add the pastina to the boiling broth, reduce heat to medium-low, and cook according to package directions, usually 4-5 minutes, stirring occasionally to prevent sticking.
When the pastina is tender and has absorbed most of the broth (but is still somewhat soupy), remove from heat.
If using egg: Beat it lightly in a small bowl, then slowly drizzle into the hot pastina while stirring constantly to create a creamy texture.
Add the butter and grated Parmigiano-Reggiano, stirring until completely melted and incorporated.
Season with salt and freshly ground black pepper to taste.
Garnish with chopped fresh parsley if desired and serve immediately in warm bowls.