Open the can of sweetened condensed milk.
Pour the milk into a saucepan.
Add the baking soda and stir well.
Heat the mixture over medium-low heat.
Stir continuously to prevent sticking.
Cook for about 1 hour, stirring frequently.
Watch for the color to change to a golden brown.
Once thickened, remove from heat.
Add vanilla extract, if using, and stir to combine.
Let it cool before transferring to a jar.