Dissolve the baking soda and cornstarch in ¼ cup (~60 mil) of milk, then combine all of the milk and sugar in the pot.

Cook, stirring frequently, over medium heat until the sugar dissolves, and then bring it to a slow boil. Watch it closely. The color will gradually change from milky white to golden brown and the consistency will thicken. Continue cooking until the mixture coats the spoon and begins to set up as it cools on the spoon, about 2 hours. Finished temperature should be 220° F/104° C. It will look runny in the pot, but will thicken nicely as it comes to room temperature.

Pour into a bowl. Don't scrape the pot or you'll have crunchy bits in the arequipe. Allow to cool, and then serve. Store in the refrigerator.