Before starting, check the farina package for its stovetop cooking time. This recipe uses plain quick-cooking wheat farina, not flavored instant packets.
In an unheated saucepan, whisk together the dry farina, sugar, ¼ teaspoon cinnamon, and salt.
Gradually pour in the cold milk while whisking until the farina is evenly dispersed and there are no dry pockets.
Place the saucepan over medium heat. Cook for approximately 5–6 minutes, whisking frequently and scraping the bottom and corners, until the mixture begins to steam and bubble gently.
Reduce the heat to low. Continue cooking at a gentle simmer for the time specified on the farina package, usually about 2½–3 minutes for quick-cooking farina. Stir almost continuously to prevent sticking and lumps.
Check that the grains are tender and the mixture falls from the spoon in a thick but flowing ribbon. If the cereal is still gritty, continue cooking briefly, adding a splash of milk if needed to keep it loose.
Stir in the butter until melted. Remove from the heat and stir in the vanilla.
If the farina has become too thick, stir in a little additional milk, 1 tablespoon at a time. It should be slightly looser in the saucepan than you want it in the bowl because it thickens as it cools.
Divide evenly between two bowls. Sprinkle the remaining ⅛ teaspoon cinnamon lightly over the servings.
Serve warm, allowing it to cool briefly to a comfortable eating temperature.