Pour the milk into a 3-quart / 2.8 L heavy-bottomed saucepan (about 20 cm / 8 inches wide) and note the level with a clean wooden spoon. Bring to a boil over medium heat, about 5 minutes, watching so it doesn't boil over. Reduce to medium-low, stir in 40 g sugar, and simmer for 25 minutes, stirring every few minutes and scraping the sides, until the level has dropped by about a third to roughly 700 ml.

While the milk simmers, put the other 40 g sugar and the water in a 16 cm / 6-inch small saucepan over medium heat. Swirl without stirring for 5 to 7 minutes until it turns deep amber, like dark honey. Take it off the heat and carefully pour in about 120 ml of the hot milk from the big pan. It will spit and seize. Return to low heat and stir for 1 minute until smooth.

Scrape the caramel milk into the big pan and stir. Take the pan off the heat and cool for 30 minutes, stirring now and then, until an instant-read thermometer reads 40 C / 104 F. Skim off any skin.

Whisk the yogurt in a small bowl with a ladle (about 60 ml) of the cooled milk until smooth. Stir it back into the pan, then pour the milk through a fine-mesh sieve into four 250 ml / 8-oz clay pots or ramekins, leaving about 1 cm / 1/2 inch below the rim.

Cover the pots loosely with foil or lids and set them somewhere warm and still, such as an oven that's off with the light on. Leave for 8 hours without moving them, until the surface is firm and the yogurt jiggles like soft custard with no liquid pooling.

Move the pots to the fridge and chill for 3 hours. Serve cold, straight from the pots.