Heat 30 ml (1 fl oz) water with the pine sap to 70°C (158°F) and cook for 20 minutes. Strain the liquid and add to the sugar in a saucepan. Cook until the mixture reaches 180°C (356°F) on a sugar thermometer, then remove from the heat and stir in the butter until melted. Add the salt and leave to set in the fridge until it has a paste-like consistency. Spoon into a piping bag and fill the chocolate shells.