Pour the milk, sugar, and baking soda into a 1.5 gallon/6 qt pot or bigger. This will bubble up some and its much nicer to have extra space.
Turn onto medium heat, whisk well to dissolve the sugar.
Keep an eye on it, stirring occasionally.
Put a freezer safe plate with a few metal spoons in your freezer for testing thickness later.
When it starts to bubble (this takes 5-10 minutes for me), turn to medium- low.
You can mostly leave it alone now when its settled into a gentle simmer.
As you'll see in the video, when the bubbles start to change, get smaller, and all over the surface, you're getting close.
When its looking real thick, take one of those spoons, take a dip out, and see how it cools down. When its room temp, is it as thick as you want it?
If on low heat, I've never burnt a batch of dulce de leche; you can get it pretty thick, or leave it thinner as a sauce. Keep in mind after it sits in the fridge it will get a bit thicker.
When it's to your desired thickness, take it off the heat and add vanilla and salt if desired.
Pour into containers or jars and keep in the fridge. I've never had it go bad, it keeps forever and ever, amen! (Well, safely a few months...)