Preheat the oven to 160C/325F. Grease a 20cm/8inch square tin and line the bottom and sides with greaseproof paper.
Put the flour, cornflour and sugar in a food processor and pulse briefly to combine.
Add the cubed butter and pulse in short bursts until the mix resembles breadcrumbs.
Alternative make by hand by putting all the ingredients together in a bowl and rubbing them together with your fingertips.
Press the shortbread into the tin until it’s packed down and level.
Bake for 35-40m. Leave to cool completely (in the fridge if necessary).
To make the caramel, put the butter, sugar and golden syrup in a pan and melt over medium heat. Add the condensed milk, bring to the boil stirring constantly. Reduce the heat to medium-low then stir for 5 minutes until caramel thickens slightly.
Stir in the peanut butter, the soy sauce and salt.
Fold the peanuts into the caramel or spread them over the shortbread in an even layer. Make sure they are quite small otherwise it will be difficult to slice the shortbread.
Pour the caramel over the shortbread layer. Put in the fridge until it sets.
Put the chocolate and butter in a bowl and then microwave for short bursts until the chocolate starts to melt. Stir until completely smooth.
Pour the melted chocolate over the caramel and level.
Heat the peanut butter and butter in the microwave for 20-30 seconds. Stir until smooth then drizzle or pipe over the chocolate in straight lines.
Give your pan a half turn and lightly draw a skewer through the peanut butter lines to create a marbled effect.
Put in the fridge for an hour or until set.
When you are ready to slice, lift the shortbread out of the tin by the baking paper. Run a large knife under hot water and wipe dry. Push firmly through the shortbread and slice it, wiping the knife between cuts.
Keep the millionaire's shortbread in the fridge and consume within 3 days.