Whisk yolks with 50g sugar and salt in a bowl until smooth; gently whisk in cream and vanilla extract. Strain custard mixture into a liquid measuring jug.
Line bottom of a large slow cooker with a folded tea towel to create a level surface so ramekins won't slide around. Set four ramekins on the tea towel. Fill slow cooker with enough water to come halfway up sides of ramekins.
Pour custard evenly into ramekins. Drape kitchen roll over top of slow cooker to absorb any condensation during baking; cover with lid to secure.
Cook on Low until custard is set but jiggles slightly, about 2 hours.
Transfer ramekins to a rack to cool completely, about 45 minutes. Refrigerate custards, uncovered, until cold, at least 3 hours.
Sprinkle 1 teaspoon sugar over each ramekin and shake gently to distribute evenly. Heat the sugar using a culinary torch until melted and browned, about 1 minute.
If you have vanilla bean paste, substitute that for more vanilla flavour.