In a large, heavy-bottomed saucepan, combine the whole milk, granulated sugar, and baking soda. Add the vanilla bean seeds and the split pod (or vanilla extract if using).
Place the saucepan over medium heat and cook, stirring constantly, until the sugar has dissolved and the mixture comes to a gentle boil.
Reduce the heat to low and continue to simmer the mixture, stirring frequently, to prevent it from sticking to the bottom of the pan. Simmer for about 1.5 to 2 hours, or until the mixture has thickened and turned a deep caramel color.
If you used a vanilla bean, remove the split pod from the mixture once it has thickened.
Remove the saucepan from the heat and let the dulce de leche cool slightly. It will continue to thicken as it cools.
Transfer the dulce de leche to a clean jar or airtight container. Once completely cooled, it can be stored in the refrigerator for up to 2 weeks.
Enjoy your meal!