Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, line a 8" square tin with greaseproof paper. I grease the tin first with a little butter to help the paper stick to it
To make the shortbread base mix the caster sugar and butter together into a bowl until creamy, using a spoon or an electric mixer with the paddle attachment
Add the vanilla and mix in
Then add the plain flour and mix in. If using a stand mixer, you may need to scrape the bowl down to make sure all of the flour is mixed in
Tip the mixture into the tin and press it down until compact and even. I find it easier to do this with my fingertips
Bake the shortbread biscuit base for 20-25 minutes until lightly golden around the edges, then leave to cool fully
To make the caramel, put all of the ingredients into a pan on a low heat and stir until everything is melted together and the sugar has dissolved
Bring the mixture to the boil, stirring continuously, and let it boil for 5-7 minutes until thick and golden. I boil mine for 6 minutes. (This instructions is for gas hobs, for induction hobs, I find that simmering for 10-12 minutes works best)
Take the mixture off the heat and stir in the vanilla. Then pour into the tin over the shortbread base
Put into the fridge to set for at least 2 hours, or overnight
Remove from the fridge and bring back to room temperature, this takes about 1 hour on the kitchen work surface
Melt both the chocolates in the microwave separately. Blast for 30 seconds, then stir, then blast in 10 second intervals stirring between each one until melted
Pour the milk chocolate over the caramel and smooth it out, tap the tin onto your work surface a few times to remove any air bubbles
Put the white chocolate into a piping bag and pipe lines on top of the milk chocolate. Then drag the tip of a cocktail stick or a knife the opposite way to the lines, up and down through the chocolate to create the featheing effect. Leave to set
The best way I've found to slice it is to firstly make sure that the chocolate has set and that it is at room temperature. If the millionaire's shortbread is cold, the chocolate is very likely to crack as you slice it. Secondly, use a very sharp knife that is longer than the shortbread. This means you can slice through it in one motion. If you like you can gently score the chocolate on top to work out the sizes of the squares first. Then, with an even pressure, gently saw the knife through the chocolate, then push it through the caramel and shortbread
Store any leftovers in an airtight container at room temperature, it will last for 2 weeks