The key to a truly delicious homemade Kahlua comes down to the quality of ingredients used. Be sure to use fresh-brewed coffee and a decent brand of alcohol for a rich liqueur that's full of flavor.
In a saucepan, bring the coffee and sugar to a boil. Reduce heat and simmer for 10 minutes. Remove from heat and allow to cool completely.
Stir the vanilla extract and alcohol of your choice into the cooled coffee simple syrup until combined.
Pour into clean bottles, cap tightly and store in a cool, dry place.
If stored properly, your homemade Kahlua will maintain its flavor and potency for about three to four years. Kahlua that has gone bad will smell off and have a watered-down taste.