Bring a large pot of generously salted water to a rolling boil. Add pastina and cook according to package directions until al dente, about 6-8 minutes.

Reserve 1 cup of starchy pasta cooking water before draining. Drain the pastina thoroughly but do not rinse.

Return the hot pastina to the pot off the heat. Add the butter and stir vigorously to begin melting.

Gradually add 1/4 cup of the reserved pasta water, stirring constantly to create an emulsified sauce. Add more pasta water 1 tablespoon at a time until the sauce is creamy and coats each grain of pasta.

Remove from heat completely. Add the grated Parmigiano-Reggiano and stir vigorously until melted and incorporated.

Season with salt and freshly ground black pepper to taste. Stir in fresh parsley if using.

Serve immediately in warm bowls with additional grated cheese on the side.