01 - Bring 4 cups of 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 yet firm.
02 - Reduce heat to low. Do not drain the pasta; allow most water to be absorbed while retaining a small amount of liquid.
03 - Stir in unsalted butter and grated Parmesan until fully melted and the mixture becomes creamy.
04 - Beat the egg in a small bowl. Slowly drizzle the egg into the hot pasta while stirring constantly to form smooth ribbons and gently cook the egg.
05 - Remove from heat, taste, and adjust seasoning with salt and freshly ground black pepper as desired. Serve immediately with additional Parmesan and parsley if using.