Preheat your oven to 170°C. Grease a 20cm square tin and line the base with baking paper.
Make the shortbread by putting the flour, castor sugar and butter into a bowl. Rub together until the mixture comes together into a dough.
Press the dough into the prepared tin in an even layer. Bake for 25 minutes until lightly golden. Set aside to cool.
Prepare the caramel by mixing the butter, condensed milk and golden syrup in a small saucepan over a medium heat. Stir until the butter has melted and the ingredients are combined. Allow to simmer gently for 6 minutes while stirring until the caramel has thickened and darkened in colour.
Remove the caramel from the heat and allow to cool for 5 minutes. Pour the caramel over the baked shortbread and spread in an even layer. Chill in the fridge for one and a half hours.
To make the ganache combine the dark chocolate and cream in a small saucepan. Stir over a low heat until the chocolate has melted and the mixture is smooth. Spread the chocolate over the chilled caramel and leave to set.
Cut the millionaire's shortbread into squares and carefully remove from the tin. Store in an airtight container.