Melt the chocolate in a thin-sided bowl in a hot bain-marie, stir in the butter. Remove the bowl, stir in the sugar. Add the egg, flour and salt, stir until smooth. Transfer the cake mixture to a piping bag, pipe into the prepared holes.
Approx. 15 mins. in the centre of an oven preheated to 180°C. Remove the cakes from the oven and allow to cool a little in the tin.