Melt dark chocolate in a bowl over simmering water without letting the bowl touch the water.
Heat chocolate to 42°C, then cool to 32°C before coating the seashell moulds.
Tidy the edges of the moulds with a scraper and save any unused chocolate.
Refrigerate the moulds for 20 minutes to set the chocolate shells.
Warm cream and muscovado sugar in a heavy-based pan over low heat.
Gently melt caster sugar in another pan until it becomes a dark golden-brown caramel.
Stir the warm cream mixture into the caramel, add sea salt, and cool until thick.
Fill each chocolate shell with a teaspoon of cooled caramel and chill until firm.
Melt white chocolate and spoon over the caramel to seal the shells.
Chill for 15 minutes then gently turn the chocolates out of the moulds.