In a large heavy-bottomed saucepan, combine the milk, sugar, and baking soda. Heat over medium-low to medium heat, stirring often, and bring the mixture to a boil.
Once the mixture begins boiling reduce heat to low and simmer, stirring often, until caramelized and thickened, about 1 ½ to 1 ¾ hours. As the mixture thickens, you will need to stir more often as it nears the end of the cooking time to avoid burning it. Watch it closely!
Remove from heat and stir in the vanilla extract.
Pour into a bowl and allow to cool to room temperature.
This made a little over 1 1/2 cups of dulce de leche.
From Sweetened Condensed Milk:
Preheat the oven to 425º Fahrenheit.
Pour the condensed milk into a ceramic pie plate or 8x8 glass baking dish. Cover with foil.
Place the pie plate into a larger baking pan.
Pour water into the large baking pan until it comes to about 3/4 of the way up the smaller baking pan.
Bake for 90 minutes or until the caramel has thickened and is light brown in color. Check it at 60 minutes to make sure the water hasn't evaporated too much. Add more water to the pan if needed.
The dulce de leche is ready to use as soon as it comes out of the oven, or you can cool it slightly. This sweetened condensed milk technique made a little over 1 cup of dulce de leche.
Store in an airtight container in the refrigerator for up to two weeks.