Whisk together yolks, 50g sugar, salt and vanilla in a bowl, then gently whisk in cream. Pour mixture through a sieve into a measuring jug.
Line the bottom of an oval slow cooker with a folded piece of kitchen roll to create a level surface so ramekins won't slide around. Set 4 ramekins on top of the kitchen roll. Fill slow cooker with enough water to come halfway up the sides of the ramekins.
Pour custard evenly into ramekins. Drape pieces of kitchen roll over the top of slow cooker (to absorb any condensation during cooking), then cover with lid. Cook on Low until custard is set but jiggles slightly, about 2 hours.
Transfer ramekins to a rack to cool completely, about 45 minutes. Chill custards, uncovered, until cold, at least 3 hours.
Sprinkle 1 teaspoon of sugar evenly over each ramekin. Brown the sugar with a culinary blowtorch (or place under a hot grill under sugar has caramelised); serve immediately.