In a small bowl combine a 1/4 cup of milk, cornstarch and baking soda, stir until dissolved. Set aside.
Place the 1/3 cup sugar in a heavy bottomed medium sized saucepan and heat on medium heat. As the sugar begins to melt spread it around the bottom of the pan with a wooden spoon so that it will cook evenly. Stir occasionally to make sure it is melting.
Watch the pot carefully once the sugar starts to caramelize, it happens very quickly. Turn down the heat and allow it to come to the darkness you prefer. I like mine pretty dark, with almost a bitter flavor. it will smoke at this point, but is not yet burned.
Remove from heat and carefully add the milk, sugar, cornstarch mixture, salt and vanilla bean. When you pour the milk onto the hot caramel it will sputter, so be careful! Return to medium heat and bring to a simmer. Reduce the heat until you have a gentle simmer and cook for about 35-45 minutes.
The cajeta will be a deep caramel brown and will have reduced from 2 cups of liquid to 3/4 of a cup. It will be the consistency of thick grade B maple syrup when it is hot, but turn to smooth honey consistency as it cools.