Heat the oven to 200°C/180°C fan. Grease and line a 20cm/ 9inch square loose bottomed brownie tin.

In a large bowl, mix the flour, caster sugar & cornflour. Use your fingertips to rub the butter into the flour mixture until well combined and it forms a dough. Then press the dough firmly into the tin.

Poke a fork throughout the shortbread, then place in the oven for 30 minutes. The shortbread should be golden brown and firm to touch. Remove from the oven and leave to cool in the tin.

Meanwhile pour the caramel, butter & sugar into a large saucepan over a medium heat. When the butter has melted, increase the heat and bring the mixture to a hard rolling boil all the while stirring continuously. Heat until the sugar thermometer reads 116°C. If you don't have a thermometer to hand, the caramel is ready when it has darkened and thickened.

Reduce the heat to a simmer & continue to cook the caramel for 5-7 minutes, stirring continuously. Pour the caramel over the shortbread and place in the fridge to set for approximately 45 minutes.

To make the chocolate topping, add the chocolate to a heatproof bowl placed over a simmering saucepan of hot water. Pour the melted chocolate over the set caramel.

Sprinkle over the chopped nuts and flaky sea salt. Leave to set in the fridge for 1 hour.

To serve, carefully remove the tin from the loose bottom and use a hot knife to slice into 9 large squares.