In a heavy saucepan, combine the syrup, sugar, salt and lemon juice and cook over medium heat, stirring until the sugar dissolves completely and the mixture begins to simmer around the edges. Wash any sugar and syrup from the sides of the pan with a pastry brush dipped in water. Cover and cook for about 3 minutes.
Uncover the pan, attach a candy thermometer to the edge and and cook uncovered, without stirring, swirling the pan once in awhile until the mixture reaches 305°F. Meanwhile, bring the cream to a simmer in a small saucepan; turn off the heat and set aside.
When the sugar mixture reaches 305°F, turn off the heat and stir in the butter, then whisk in the hot cream; it will bubble up and steam. Turn the burner back on and bring the mixture to a boil, stirring. Remove from the heat and set aside to cool completely.
Store in the fridge; when ready to serve, reheat (if you like) and drizzle over ice cream, cake or your finger; sprinkle lightly with flaky salt immediately before serving.