Pull a fresh espresso into a warmed espresso cup.
Pour the grappa straight in and do not stir.
Drink it in two mouthfuls, standing up, as intended.
Standard across northern Italy, ordered by name at the bar and drunk on your feet. Sambuca and brandy are the other two accepted corrections. Some bars pour the grappa into the empty cup afterwards instead — that version is called a resentin, and it is worth doing at least once.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Caffè Corretto.