First, make the caramel biscuit. Grease 2 x 22cm diameter sandwich cake tins and line the bases with baking paper. Put the biscuits into a food processor and whizz to fine crumbs. Put the sugar and water into a heavy-based saucepan and dissolve over a lot heat. Increase the heat and cook until it forms an amber caramel.
Remove from the heat and carefully stir in the cream, followed by the butter and salt. Stir the biscuit crumbs into the caramel, and then divide equally between the prepared tins and press down with the back of a spoon. Place in the freezer to set.
To make the sponge, pre-heat the oven to 180°C.
Grease a deep 22cm diameter loose-bottomed cake tin and line with baking paper.
Melt the chocolate, butter and milk together in a heatproof bowl set over a saucepan of barely simmering water, making sure the surface of the water does not touch the bowl.
In a large bowl, whisk the egg yolks and sugar together using an electric hand whisk, until pale and fluffy. Stir in the chocolate mixture, then fold in the flour. In a clean, dry bowl, whisk the egg whites until the form soft peaks, then gently fold into the chocolate mixture.
Remove the caramel biscuit discs from the freezer and remove them from the tins, discarding the lining paper. Place one of the discs on the bottom of the prepared loose-bottom cake tine and spread with half of the cake mixture. Lay the other biscuit disc on top and cover with the remaining cake mixture.
Bake in the oven for 25-30 minutes until the cake is just cooked – it should be almost undercooked for extra gooeyness. Leave to cool in the tin for 10 minutes, then turn out onto a cooking rack to cool completely.
To make the glaze, put the chocolate into a heatproof bowl.
Put the cream into a saucepan and heat to simmering, then pour one quarter over the chocolate. Leave for 1 minute until the chocolate starts to melt, then gently stir in the rest of the cream until smooth and glossy.
Cover the cooled cake with the chocolate glaze and use a palette knife to spread it evenly over the top and down the side. Lightly sprinkle sea salt crystals on top of the cake, and chill it in the refrigerator until set before serving.