Pour the sweetened condensed milk into your heavy-bottomed pot, then add the baking soda and stir quickly with a wooden spoon. The mixture will immediately foam up because the baking soda reacts with the acidity in the milk, which helps darken the color faster and develop deeper flavor.

Place the pot over medium heat and bring the mixture to a gentle boil, stirring occasionally to prevent sticking on the bottom. The foam from the baking soda reaction will settle down within the first minute or two.

Once boiling, reduce the heat to medium-low so the mixture simmers gently and steadily without rapid bubbling. A gentle, consistent bubble rate prevents scorching while allowing the milk solids to caramelize properly.

Stir the mixture every 5 to 10 minutes using a wooden spoon, reaching the corners and bottom of the pot to prevent scorching and ensure even caramelization.

After about 20 to 30 minutes of simmering, observe the mixture as it changes color from pale ivory to light tan, then gradually to a deeper caramel brown, indicating caramelization of milk sugars.

At around 45 to 50 minutes of cooking time, check the consistency. The mixture should coat the back of a spoon and drip off slowly. If using a candy thermometer, aim for 220 to 225 degrees Fahrenheit for a thick but pourable consistency.

Remove the pot from heat and stir in the vanilla extract and a pinch of sea salt, mixing well to distribute evenly. Adding vanilla off the heat preserves its delicate flavor.

Allow the cajeta to cool for 10 to 15 minutes in the pot, during which it will thicken slightly. Transfer to clean airtight glass jars after reaching room temperature. The sauce will thicken further as it cools completely.