01 - Bring water to a boil in a medium saucepan. Add kosher salt and pastina. Stir and cook for 4 to 5 minutes, until pastina is tender but not mushy.
02 - Lower the heat to low. Leave the pastina in the pot with a bit of liquid remaining; do not drain.
03 - Stir in butter and grated Parmesan until fully melted and creamy.
04 - Beat the egg in a small bowl. Slowly drizzle the beaten egg into the hot pastina while stirring constantly to create silky ribbons and gently cook the egg.
05 - Remove from heat. Adjust seasoning with salt and freshly ground black pepper if desired. Serve immediately, topped with additional Parmesan and parsley if using.