Preheat the oven to 180C/350F/Gas 4 10 minutes before baking. Lightly oil and line an 18 cm x 28 cm tin with greaseproof or baking paper.
Cream together the 125 g butter, peanut butter 75 g caster sugar until light. Sift in the cornflour and flour together and mix in to make a smooth dough.
Press the mixture into the prepared tin and prick all over with a fork. Bake in the preheated oven for 20 minutes, until just golden. Remove from the oven.
Meanwhile for the topping, combine the 200 g caster sugar, 125 g butter, golden syrup, glucose, water and milk in a heavy based saucepan.
Stir constantly over a low heat without boiling until the sugar has dissolved. Increase the heat, boil steadily, stirring constantly, for about 10 minutes until the mixture turns a golden caramel colour.
Remove the saucepan from the heat and add the pecans. Pour over the shortbread base immediately. Allow to cool, then refrigerate for at least 1 hour.
Break the chocolate into small pieces and put into a heatproof bowl with the 1 tbsp butter.
Place over a saucepan of barely simmering water, ensuring that the bowl does not come into contact with the water. Leave until melted, then stir together well.
Remove the shortbread from the refrigeratror and pour the chocolate evenly over the top, spreading thinly to cover. Leave to set, cut into squares and serve.