Preheat the oven to 175°C (350°F) and line the bottom and sides of a 12X9 inch rectangular baking pan with parchment paper.
Place the butter into a mixing bowl with the paddle attachment and cream by beating the butter on medium speed for 2 minutes. You can also use a hand mixer to follow the same instructions.
Add in the icing sugar and continue to beat until light and creamy.
Now add the flour and mix on slow just until the shortbread dough comes together.
Transfer the cookie dough onto the lined baking tray and evenly spread it out throughout the pan.
Place the pan into the freezer for 15 minutes or refrigerator for 30 minutes so that the pressed shortbread dough firms up.
In the meantime prepare the caramel sauce, add the sugar into a pot, and heat on medium-high. The sugar will begin to melt after about 2 minutes, using a whisk stir until the sugar is completed melted, then lower the heat to a low medium setting and cook until the sugar reaches a dark amber color.
Add in the butter and stir using the whisk until the butter has melted.
Remove the pot from the heat, wait a couple of seconds then slowly pour in the heavy cream stirring constantly until the caramel is smooth and begins to slightly thicken. Set aside, you can prepare the caramel sauce ahead of time, keep it in an airtight mason jar refrigerated for up to three weeks. Warm-up before using.
Remove the shortbread dough from the refrigerator or freezer and using a fork prick holes throughout the surface.
Place into oven and bake for about 20-22 minutes or until the shortbread crust is golden brown.
Remove from the oven and cool in the tray for about 30 minutes.
Make the ganache, Add the chocolate chips into a medium-size bowl and set aside.
Pour the heavy cream into a pot and bring to a simmer on medium-low heat setting. Once the cream begins to simmer, carefully pour it over the chocolate chips and stir with a whisk starting from the center in a swirling motion to outward.
Continue to stir until ganache is smooth and velvety.
Millionaire shortbread assembly spread an even layer of caramel onto the top of the shortbread with an offset spatula.
Place into the freezer for 15 minutes so that the caramel hardens then remove.
Pour the ganache over the caramel and evenly spread it throughout the surface.
Return back to the freezer for 15 minutes or refrigerator for 30 minutes so the ganache sets.
Remove, cut into squares or rectangular bars and serve.