Start by making the caramel sauce. Add the sugar to a pan with the water. Place on a medium heat, and leave for the sugar to melt away and it starts to bubble. You don't mean to stir the mix. Once the sugar has darken in colour slightly stir in the butter. Take off the high and stir through the cream. Set this aside in the fridge to cool. There will be enough for you to have it another day too.
Temper the chocolate and leave to cool slightly. When as a pourable consistency but not too runny, spoon inside the domed silicone moulds. Let it cool slightly so the chocolate doesn't all sink to the bottom of the mould. You then want to carefully move the chocolate around the moulds by moving it side to side, making sure the sides are totally covered. There will be excess chocolate so let that drip out too. Using a palette knife scrape off the extra chocolate from the moulds and set upside down on a tray. Leave upside down, in a cool place to set slightly and for the extra chocolate to drip out. Whilst you are waiting for this, melt the marshmallows. This can be tricky as they set fast, but if this happens you can keep remelting. When the chocolate is cool enough, fill them into the chocolate shells, making sure they aren't going to melt them. Top with the suitably sized biscuit, pushing down to flat the marshmallow and so the biscuit lies flat and then pour in the caramel sauce. Very carefully, pour over the final layer of chocolate, using a palette knife to flatten so the chocolate is completely even. Leave in a cool place to set, overnight would be ideal. Pop out and enjoy!