Cream-colored, simply sweetened condensed milk will develop a deep butterscotch tone and complexity when it undergoes caramelization as well as the Maillard reaction, the heat-induced interplay between sugars and proteins that causes foods to brown and take on rich, toasty flavor. But the dramatic shift in its consistency—from treacly fluid to jammy goo—is due to a separate change to the milk’s proteins. Given ample heat (starting around 170 degrees) and time (an hour or more), they denature and form a gel that gives dulce de leche its semisolid body.
Surprisingly, evaporation plays a minimal role here; most of the milk’s water is removed during the condensing process, and much of what’s left is bound up in the mixture’s high concentration (about 50 percent by weight) of sugar. In fact, the thickest dulce de leche is made when there’s no evaporation at all: in a multicooker’s pressurized chamber, where the milk reaches 240 degrees and forms an exceptionally stiff gel. In the oven, water in the milk can’t rise above its natural boiling point, so its proteins link up less thoroughly, and the jam is a little looser.
It’s important that these reactions occur steadily, lest the milk brown unevenly or curdle. (I omitted the dash of baking soda that many recipes call for, since the alkaline agent raises the pH of the milk, catalyzing the browning reactions at a rate that caused my dulce de leche to easily overcook.) The steam-filled multicooker heats evenly, while using a water bath in the oven steadies the milk’s temperature so that it cooks consistently.
Stirring in vanilla and salt makes the butterscotch flavor pop.