Heat ½ cup (125 mL) of the butter, the brown sugar, and the salt in a heavy-bottomed saucepot over medium heat, stirring occasionally. Bring to a full boil and boil for 4 minutes, just stirring once or twice. Whisk in the cream and boil until it reaches 245°F (118°C) on a candy thermometer. Remove from the heat and cool for 30 minutes, stirring once or twice.