Grease a 15 × 20 cm (6 × 8 inch) baking tray with butter.
Mix the salt into the butter and chill in the refrigerator. Melt the chocolate in a bowl over a double boiler. In a saucepan over medium heat, bring the sugar, glucose and 40 ml (¼ fl oz) water to the boil and continue cooking until the mixture turns a light caramel colour (make sure you do not stir the mixture). Remove the mixture from the heat and stir in the salted butter and cream. Return the pan to the heat, bring the mixture to the boil again and cook to 118°C (244°F) using a sugar thermometer. Remove from the heat, add the melted chocolate and mix until just combined. Pour the caramel into the prepared tray, allow to set at room temperature, then refrigerate. When the caramel has set, cut it into squares or your desired shape and sprinkle with vanilla salt.