Pour the sweetened condensed milk into a heavy-bottomed saucepan.
Add baking soda to the milk and stir to combine.
Place the saucepan over medium heat and bring to a gentle simmer.
Stir the mixture continuously using a wooden spoon.
After 10 minutes, reduce the heat to low and continue stirring.
Cook for 1 to 1.5 hours, or until thickened and golden brown.
Remove from heat and stir in vanilla extract and a pinch of salt.
Let it cool slightly before transferring to a glass jar.
Cool completely at room temperature.
Store in the refrigerator for up to two weeks.