Preheat the oven to 350°F. Line a 9x13 straight-sided metal baking pan with parchment paper allowing the parchment to overhang the sides of the pan slightly. This will allow for easy removal once chilled. Lightly spray the parchment with nonstick baker’s spray and set aside.

Cream together in a large bowl, on medium-low speed using a handheld mixer, the butter and granulated sugar for 1-2 minutes or just until smooth.

Add the egg yolks and vanilla extract. Beat on low speed until fully incorporated.

Add the all-purpose flour and salt. Beat again until just combined and no dry pockets of flour remain. You do not want to overmix the dough at this point. You are looking for the mixture to still be quite crumbly and not come together into a dough ball. This will make it easier to spread into the prepared baking pan.

Transfer the mixture into the prepared baking pan. Using your fingers, or a second piece of parchment paper placed onto the top of the mixture, gently press the dough into an even layer.

Bake the crust for 22-24 minutes or just until lightly golden around the edges. Remove the crust from the oven and let it cool while preparing the caramel layer.

To a 3 ½ - 4 quart heavy bottomed saucepan over medium heat, add the butter, sweetened condensed milk, light brown sugar, and corn syrup. Gently stir while the butter melts, until the mixture is smooth and evenly combined.

Allow the mixture to come to a low boil then reduce the heat to medium-low and cook the caramel, while gently stirring constantly to prevent burning, until the temperature of the caramel reaches 225°F on a candy thermometer and lightly golden in color. This can take up to 15 minutes depending on your saucepan and stovetop. Do not overcook the caramel or it will be too tough to chew once chilled. It is best to go by the temperature and not the time for this step.

Remove the saucepan from the heat and immediately whisk in the salt and vanilla extract until fully combined and the caramel is smooth.

Pour the caramel over the crust and smooth to an even layer. Let the caramel layer cool at room temperature for at least 30 minutes before adding the ganache to allow the caramel to firm up so the layers do not mix.

To a microwave safe bowl, add the semi-sweet chocolate chips and heavy cream. Heat for 1 minute on high, stir and heat in 30 second increments (stirring between each) until the chocolate is smooth.

Stir in the corn syrup until fully combined.

Pour the ganache over the caramel and smooth to an even layer. Allow the ganache to cool for 10-12 minutes at room temperature before sprinkling on the flaky sea salt for garnish. This allows the ganache to set up slightly so that the sea salt does not immediately melt into the warm chocolate.

Chill the millionaire’s shortbread for 1 ½ - 2 hours or until the ganache has firmed up enough to slice.

Gently lift the shortbread out of the pan, using the overhanging parchment, and slice into 24 squares using a long sharp knife. You can run the knife under hot water, then wipe dry, between cuts for nice clean slices that show each layer clearly. If refrigerated for longer than 2 hours, you may need to allow the shortbread to sit at room temperature for 10-15 minutes before slicing and serving.