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 biscuit base mix the caster sugar and butter together into a bowl until creamy, using a spoon or an electric mixer if you have one
Add the plain flour and mix in until a dough forms
Arrange the halved mini creme eggs in the bottom of the lined tin, press the shortbread base down with your fingers over and in between them until compact and even
Bake the shortbread biscuit base for 20-25 minutes, until lighty golden, then leave to cool fully in the tin
To make the caramel, put all of the ingredients into a pan on a low heat and stir until everything is melted together
Bring the mixture to the boil, stirring continuously, and let it boil for 5-7 minutes until thick and golden (I boiled mine for 6 minutes)
Take the mixture off the heat, then pour the caramel over the top of the shortbread base
Put into the fridge to set for at least 2 hours, or overnight if you prefer
Melt the dark and milk chocolates in the microwave. Break into pieces and put in a microwave safe bowl. Blast for 30 seconds, then stir, then blast in 10 second intervals stirring between each one until melted
Pour the chocolate over the caramel and smooth it out, tap the tin onto your work surface a few times to remove any air bubbles and to create an even surface
Press the creme egg and mini creme egg halves into the chocolate and add sprinkles if using
You can set the chocolate in the fridge for a quicker result, or leave at room temperature to set
It is best to slice this shortbread at room temperature to avoid cracking the chocolate top, remove the millionaire's shortbread from the tin and remove the baking paper. Slice carefully using a large sharp knife, it helps if the length of the knife is longer than the shortbread
Store any leftovers in an airtight container, kept in the fridge it will last for 2 weeks