Crush the biscuits in a food processor to give fine crumbs. Melt 100g butter and mix in the crumbs.
Press into the base and sides of a 4cm deep, 20cm round loose bottomed tin and bake for 10 minutes. Cool.
Melt the remaining butter and sugar in a medium saucepan and gently cook for 3 minutes, stirring constantly. Add the condensed milk and bring to the boil, stirring and cook for 1 minute, stirring all the time to give a thick golden caramel.
Stir in the LoSalt and pour into the tin. Chill for at least 4 hours or overnight.
Spread the bananas on top of the caramel and top with the yoghurt, remove from the tin. Sprinkle with cocoa and serve.