In a 5 litre (175 fl oz/20 cups) cast-iron casserole or saucepan, bring the milk to a boil with the sugar and scraped vanilla bean and seeds, stirring to dissolve the sugar.
Add the bicarbonate of soda and reduce heat to low. Continue cooking for 2.5 –3 hours, stirring with a wooden spoon from time to time. It will darken and thicken.
Use the cold plate test to check whether the dulce de leche is ready, put a spoonful on a cold plate and tilt it.
If the mixture holds its shape, remove it from the heat; if it runs a little, cook until it reaches this stage.
Store in an airtight jar in the refrigerator for up to 10 days.
Recipe and image from Argentinian Street Food, Enrique Zanoni & Gaston Stivelmaher, Murdoch Books.