Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, line a 8" square tin with greaseproof or baking 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, ideally using an electric stand mixer with the paddle attachment
Add the vanilla extract and mix in
Then add the plain flour and salt (if using) 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, but the back of a spoon works too
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 except the vanilla extract 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 extract. Then pour it 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 feathering 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 for 5-7 days, kept in the fridge it will last for up to 2 weeks
Video
Notes
Make sure your butter is at room temperature before you start baking the shortbread.
I used unsalted butter for both the shortbread and caramel layers. You can use salted butter if you like, it won't make it taste salty, instead it will balance out the sweetness. However, I do prefer to use unsalted butter and add my own salt as then I can control the salt levels. You can also use a baking spread, but butter tastes much better in my opinion. It's not that baking spread tastes bad, just that real butter is richer and creamier.
You must use plain flour (also known as all purpose flour) for the shortbread, self raising flour will make the shortbread rise, which you don't want to happen.
You must use sweetened condensed milk for the caramel layer, evaporated milk will not work.
I use granulated sugar for the caramel layer, but caster sugar (also known as superfine sugar) works well too.
Golden syrup is sold all over the world, so check the international section in your supermarket if you’re not based in the UK. You can also buy golden syrup online from Amazon. If you can’t get hold of it, light corn syrup is the closest thing to it. You can also use light brown sugar, your caramel will look different and have slightly different texture – but it will still be delicious! Check the recipe notes for details.
You can use any kind of chocolate for the topping. Milk chocolate is the traditional option, but dark and white chocolate work well too. Pick any chocolate that is good quality and that you like the taste of, I'd advise avoiding cooking chocolate as it doesn't taste great.
Although I provide cup measurements, I highly recommend weighing your ingredients out using digital kitchen scales. It is the most accurate way to measure ingredients and will ensure the best results. Digital scales are very low cost and can be purchased for around £12 ($16.50).
For teaspoon (tsp) and tablespoon (tbsp) measurements, please use measuring spoons and not the type of spoons you eat with. Again this will ensure accuracy and provide the best results.
If you can't get hold of golden syrup you can make the caramel in the same way using these ingredients: