You can buy cajeta
online or find it in well-stocked grocery stores. Dulce de leche (or
confiture de lait, as it's known in France) is made with cow's milk, and is a delicious substitute. You can make your own
Dulce de leche as well.
When layering the cajeta in the just-churned ice cream, it should be pourable; about the consistency of ketchup. Warm it gently by putting the jar in a small saucepan of warm water or in the microwave for about 10 seconds, just until it's pourable. Let it cool to room temperature, stirring, before layering it with the ice cream.
I haven't made this with any of the non-dairy "milks" out there (such as almond, soy, rice, or oat) but if you do try it, let me know how it turns out in the comments.
Serving: Serve in bowls. You can garnish scoops of the ice cream with additional cajeta, if you wish.