To make the ganache, heat the double cream and butter in a saucepan until just below a simmer. Place the chocolate into a heatproof bowl and once the cream is ready, pour it over the chocolate and allow to sit for a couple of minutes before stirring to combine fully. Place in the fridge and allow to set.
Grease a 9-inch tart tin lightly with a little butter.
In a small saucepan, melt the butter.
Blitz the biscuits with cocoa powder until they are crumbs and then blitz gently as you pour in the butter.
Spoon the mixture into a 9inch fluted tart tin and smooth firmly into the sides and base. Allow to freeze to set for 5 minutes.
To make the caramel, place the caster sugar in a small saucepan with a little water and heat gently to dissolve the sugar, do not stir.
Once the sugar has dissolved, turn the heat up and bubble until you have a deep amber colour. Acting quickly and carefully, pour the double cream into the pan and swirl to combine. Add in the butter and salt and then swirl again to combine. Allow to stop bubbling and cool slightly then pour into the tart tin and place in the fridge to cool completely and set for 10 minutes.
Meanwhile, whisk the set ganache with a electric hand mixer until light and fluffy.
Once the caramel is cool, remove from the fridge and spread on the whipped ganache, leaving a small border to see the caramel. Sprinkle over a small pinch of sea salt flakes.
Remove from the tin and serve.