In a small dish, dissolve the baking soda in a couple Tablespoons of the milk and set aside.
Get out a very large pot (the mixture will bubble and foam, so you need a pot at least twice the size you think. I used a 6 qt dutch oven.) Stir together the remaining milk and sugar in the pot. Add the vanilla bean pod if using.
Over medium heat, bring the mixture to a low boil, stirring frequently. Turn off heat and stir in the dissolved baking soda mixture. Be careful, this is one of those bubbling like crazy moments!
Return to medium heat and cook, stirring frequently, until the mixture is golden and thick like honey. This will take about 45 minutes to an hour. You don’t need to stand over it the whole time, but you’ll want to keep an eye on it as it will bubble and you don’t want it sticking to the bottom of the pan.
Remove from heat and remove vanilla bean if you used it. If not, stir in vanilla extract and let cool.
Store in airtight container in the refrigerator for up to a couple of weeks. It will tighten up in the cold, but can be made pourable again with a short stint in the microwave.
Notes
For a little extra flavor, you can steep a cinnamon stick in the mixture while it cooks. Remove it when you are done cooking.