In a large pot over medium heat, stir together milk, sugar, and baking soda.
Bring to a boil, stirring occasionally, making sure to remove the pot from the stove if it looks like the mixture is going to foam over. Keep stirring until the foam subsides and then return to the stovetop.
Once the mixture is boiling consistently without foaming, reduce the heat and simmer uncovered for about 1 to 1 1/2 hours, stirring occasionally, until the milk has caramelized and thickened.
When the mixture is a beautiful golden brown and the mixture has thickened to where it sticks very well to the back of a spoon, remove from heat and stir in vanilla extract and ground cinnamon.
Transfer to a glass jar to cool and enjoy!
For the easy 1-ingredient version
Preheat the oven to 425°F. Pour the sweetened condensed milk into a 9-inch baking dish (I used a pie plate) and cover it tightly with foil.
Place the covered dish in a larger pan (like a roasting pan) and pour hot water into the larger pan until it comes halfway up the side of the 9-inch dish.
Carefully transfer it to the oven and bake for 1 hour and 45 minutes.
Remove the 9-inch baking dish from the water bath and carefully remove the foil. Whisk the dulce de leche until completely smooth and transfer it to a glass jar to cool. Enjoy!
Makes about 1 1/4 cups.Store dulce de leche in an airtight container in the fridge for up to one month.For the easy version, make sure to check the water level in the roasting pan halfway through baking and add more hot water if needed. If the dulce de leche has not reached a beautiful golden brown color after the time is over, let it bake for an additional 15 minutes.