Not everyone has all of the ingredients required in a recipe--especially rarely used foods like evaporated milk. Evaporated milk has had half of the water removed, making it a denser product than milk, according to Practically Edible. Replacing evaporated milk with buttermilk will result in a less creamy product, but you can successfully make the substitution in your cooking. Buttermilk has a slightly acidic flavour to it, which requires compensation by adding baking soda to your recipe to keep the balance of acids and bases correct.
Look at your recipe for the amount of evaporated milk required and double that number to determine the amount of buttermilk to use. For instance, if your recipe calls for ½ cup of evaporated milk, use 1 cup buttermilk.
Fill a shallow skillet with twice the evaporated milk required and heat over low heat until the mixture reduces by half. For instance, if you heat 1 cup buttermilk, heat it until it reduces to ½ cup of buttermilk.
Remove the milk from the heat and let cool.
Stir ½ tsp baking soda into every 1 cup of buttermilk originally added to the skillet. For example, add ½ tsp of baking soda to ½ cup of reduced buttermilk which originally measured 1 cup of buttermilk.
Add this evaporated milk replacement when your recipe calls for the addition of the evaporated milk.