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 light brown soft sugar, Biscoff spread 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 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 instruction 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 into the tin over the shortbread base
Leave to fully cool and set, you can speed this up by putting it in the fridge, but bring it back to room temperature before you add the chocolate
Melt the chocolates in the microwave. 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
Warm the Biscoff spread in the microwave for 20-30 seconds to make it pourable, then drizzle it all over the white chocolate. Use a cocktail stick to swirl the Biscoff spread around
Decorate with the Biscoff biscuits. Leave to set at room temperature
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