In a medium-large (6-quart) pot (preferably a Dutch oven or Mexican copper cazo), combine the milk, sugar and cinnamon stick and set over medium heat. Stir regularly until the milk comes to a simmer (all the sugar should have dissolved by this point). Remove the pot from the heat and stir in the dissolved baking soda—it’ll foam up if the goat’s milk is acidic. When the bubbles subside, return the pot to the heat.
Adjust the heat to maintain the mixture at a brisk simmer (too high and the mixture will boil over; too low and the cooking time will seem interminable). Cook, stirring regularly, until the mixture turns pale golden, more or less one hour.
Now, begin stirring frequently as the mixture colors to caramel-brown and thickens to the consistency of maple syrup (you’ll notice the bubbles becoming larger and glassier). Stir regularly so nothing sticks to the bottom of the pot. Test a couple of drops on a cold plate: When cool, the cajeta should be the consistency of a medium-thick caramel sauce. If the cooled cajeta is thicker (almost like caramel candy), stir in a tablespoon or so of water and remove from the heat; if too runny, keep cooking.
Pour the cajeta through a fine-mesh strainer set over a bowl or a wide-mouth storage jar. When cool, cover and refrigerate until you’re ready to serve. Warming the cajeta before serving (a microwave oven is efficient here) makes it extra delicious.
Working Ahead: Cajeta keeps for a month or more in the refrigerator. Keep it tightly covered to keep it from absorbing other flavors.