Fill a double old fashioned glass with ice cubes.
Pour 50ml of Fernet-Branca over the ice.
Fill the glass with cola (approximately 150ml), pouring slowly down the side of the glass to preserve carbonation.
Gently stir once or twice from the bottom to combine.
The Fernandito is the national drink of Argentina, where Fernet-Branca consumption per capita exceeds that of any other country in the world - Argentines drink it so frequently that Fernet-Branca built a dedicated production facility in Córdoba. The classic ratio is roughly 1:3 Fernet to cola, though locals often go bolder at 1:2.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Fernandito.