Stir together the milk and sugar in a large, heavy-bottomed pot. (Make sure the liquid only goes three fourths the way up the sides, as it will froth when the baking soda is added.) Add the cinnamon stick, salt, vanilla extract and vanilla bean seeds plus the empty vanilla bean pods. Bring to a boil on medium-high heat while constantly stirring. This will take about 15 minutes. DO NOT take your eyes off the pot. It boils over easily and quickly!