Combine the milk, sugar, and bananas in a large, heavy-bottomed saucepan.
Bring to a boil.
Add the baking soda and reduce the heat to medium, stirring until the foam has decreased, about 15 minutes.

Let simmer, stirring occasionally to prevent solids from forming in the foam. (It may be necessary to skim the dulce de leche after 30 minutes.) Continue cooking until the mixture starts to thicken (the entire process can take up to two hours, depending on the width of the saucepan and the heat of the stove).
Once the foam and the milk are almost the same color, stir constantly for about 15 minutes or until you can see the bottom of the saucepan when you stir the mixture with a wooden spoon.
This will become very thick and spreadable.
Remove from heat, add the rum and vanilla seeds, and mix.
Place in jars and seal.
Sterilize for 15 minutes in boiling water. The jars will keep for 6 months in the refrigerator.