Ingredients

Step by step

1. First make the caramel biscuit. Grease 2 x 18cm diameter sandwich cake tins and line the bases with baking paper. Put the biscuits into a food processor and whiz to fine crumbs. Sprinkle the sugar into a heavy-based saucepan in an even layer and dissolve over a low heat, keep the pan over the heat until you end up with an amber caramel, give it a stir from time to time. Remove the pan from the heat and carefully stir in the cream, the caramel will bubble but keep stirring, it will come together again. Next add the butter and salt. Stir the biscuit crumbs into the caramel, then divide equally between the prepared tins and press down with the back of a spoon. Place in the freezer to set for 2 hours.
2. To make the sponge, preheat the oven to 180°C, fan 160°C, gas 4. Grease a deep 23cm diameter loose-bottomed cake tin and line it 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.
3. 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 to soft peaks, then gently fold into the chocolate mixture.

4. 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-bottomed cake tin and spread with half of the cake mixture. Lay the other biscuit disc on top and cover with the remaining cake mixture.

5. Bake in the oven for 1 hour until the cake is just cooked – it should be almost undercooked for extra gooeyness. Leave to cool in the tin. The cake will sink a little in the middle but that is normal.

6. To make the glaze, put the chocolate into a heatproof bowl with the golden syrup. Put the cream into a saucepan and heat to simmering point, then stir in the golden syrup (if using), pour over the chocolate. Leave for 1 minute then gently stir in the rest of the cream until smooth and glossy.
7. 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, leave to set before serving.