Dissolve the sugar into the water in a heavy bottomed saucepan and let it bubble away over a medium heat.
The mixture should turn a golden caramel colour after around 5 minutes. DON'T stir the caramel mixture - the sugar will crystallize.
Once the caramel has reached the desired colour, take it off the heat and add the salt and cream. Stir furiously as the caramel will want to harden quickly.
Pop the caramel cream mixture in a small bowl and put it in the fridge to help cool down quickly.
Using an electric mixer, cream the butter and icing sugar for 5 minutes until fluffy.
Add the cooled caramel cream mixture and continue beating until well combined.
Dollop a generous amount of the buttercream (about 1 tablespoon) on half of the cooled cookies and sandwich the cookies together with the remaining ones.