For the base, preheat the oven to 160C/310F/Gas 3. Grease a 20cm x 23cm/8in x 9in baking tin with butter.
Sift the flour, cornflour and sugar into a large bowl. Stir in the almonds, then rub in the butter and vanilla seeds.
Press the mixture into the base of the prepared tin. Transfer to the oven and bake for 35-40 minutes, or until lightly golden. Allow to cool.
For the caramel, place the sugar and water into a heavy-based pan and heat gently until the sugar dissolves. Turn up the heat and simmer for 4-5 minutes, or until it turns dark amber.
Stir in the cream, taking care as it will bubble up. Stir in the butter and salt. Pour the caramel over the shortbread and leave to cool.
For the topping, melt the chocolate and butter in a heatproof bowl set over a pan of gently simmering water.
Pour the chocolate over the caramel and smooth out with a wet palette knife. Place into the fridge and chill for at least 30 minutes, until firm. Cut into 16 pieces and serve.