Put milk, sugar and vanilla bean paste (or vanilla bean pod and seeds) in a large sauce pan over medium heat and bring to a simmer stirring occasionally.
When sugar completely dissolves, add baking soda and salt. Reduce to low heat and cook for 3 ½ to 4 hours uncovered, stirring occasionally. If using vanilla bean pod and seeds, you’ll need to remove the pod after cooking for an hour.
Skim off foam as necessary (if there is too much).
In the last hour of cooking, stir the mixture frequently and reduce heat to the lowest setting if necessary, the mixture will bubble more rapidly.
Reduce the mixture to about 1 ½ cups. When it thickens and the color is caramel brown, it is ready.
Transfer to a jar and let cool to room temperature.