A rich, creamy caramel sauce made by slowly simmering sweetened milk until it thickens and turns golden brown. This stovetop method yields a traditional Argentine dulce de leche perfect for spreading, filling, or drizzling.
In a large, heavy-bottomed saucepan (at least 4-quart), combine milk and sugar. Scrape vanilla bean seeds into the milk and add the pod (or add vanilla extract later).
Place over medium heat and stir constantly until sugar dissolves and mixture comes to a gentle simmer, about 5-7 minutes. Do not let it boil over.
Reduce heat to low and stir in baking soda. The mixture will foam up; stir gently until foam subsides.
Continue to cook over very low heat, stirring occasionally with a wooden spoon or heatproof spatula, scraping the bottom and sides to prevent scorching. Maintain a gentle simmer (small bubbles).
After about 1 hour, the mixture will begin to thicken and turn a pale caramel color. Stir more frequently (every 5 minutes) as it thickens.
Continue cooking until the dulce de leche reaches a deep golden brown and thickly coats the back of a spoon, about 45-60 more minutes (total 1.5-2 hours). The color should be like caramel.
Remove from heat. If using vanilla extract, stir it in now. Discard the vanilla pod. Let cool for 15 minutes; it will thicken further as it cools.
Transfer to a clean jar or container and let cool completely before sealing. Store in the refrigerator.
Tips
For a quicker version, use a slow cooker: combine all ingredients in a slow cooker and cook on low for 8-10 hours, stirring occasionally.
Dulce de leche can be stored in the refrigerator for up to 2 weeks. Warm slightly before using if it becomes too thick.
Use as a spread on toast, crepes, or cookies; as a filling for cakes or alfajores; or as a topping for ice cream or flan.