This recipe is from my Argentine host mother, Carmen Murphy.
Bring milk and sugar to a boil. When it just begins to boil, add 1 T baking osda. When the milk rises (it will try to boil over), lift the pot from the burner until the milk falls again. Repeat until the milk stops rising. Cook until done. To test for thickness, place some dulce de leche on a plate and wave it in the air so that it cools. Watch how fast it slides down the plate.