Heat the oven to 160°C (fan 140°C, gas mark 3). Line a 20cm x 23cm tin with baking paper, making sure the paper overhangs slightly so it makes it easier to remove finished cake .
For the shortbread layer, place the sugar, butter and flours into a food processor, whiz until soft dough is made. Spoon into the prepared tin and press lightly to form a smooth base.
Chill for about 20 minutes and then bake for 35 minutes or until golden. Leave to cool.
To make the caramel, place the butter, sugar, syrup and condensed milk into a saucepan and stir over a low heat until the butter has melted.
Bubble and stir the mixture gently for five to eight minutes, until you get a thick and fudge-like mix. Pour over the cold shortbread in an even layer. Leave to cool.
Once the caramel has cooled, chop the chocolate and place in a bowl suspended over a pan of simmering water. Do not let the water touch the bottom of the pan. Stir the chocolate until melted. Spread evenly over the caramel and allow to set.
When ready to serve remove from the tin using the paper to help and cut into little squares.