Preheat oven to 350F with the rack in the middle, line a 9×9 inch baking dish with parchment paper.
In a large bowl, beat together butter and sugar with an electric mixer on medium until fluffy, about 2 minutes. Add the vanilla and egg yolk then mix until combined. Gradually add the flour and salt, and beat on low just until the mixture is evenly combined and the texture of coarse sand.
Tip mixture into prepared pan, and press down firmly into an even layer. Bake until top is dry and light golden brown about 22 minutes. Transfer to a rack to cool.
While the shortbread is baking, make the caramel. This is easiest with an instant read candy thermometer, but can be done without. In a medium saucepan, combine the butter, brown sugar, salt, vanilla, sweetened condensed milk and corn syrup. Cook mixture over medium heat, whisking constantly. Mixture will thicken & change from a softer yellow color to a darker one, this taking about 6 minutes minutes. The thermometer should read 220 degrees to 225 degrees. Adjust the heat as necessary to keep the bottom from scorching. You will see the caramel start to thicken and pull away from the edge. Immediately pour the caramel over the baked shortbread crust and spread it into an even layer making sure to reach the edges. Chill for about 10 minutes so the caramel sets.
Chop chocolate & melt with cream either in a double boiler or in the microwave. For the microwave melt in short 20 second bursts stirring in between. Spread the ganache in an even layer over caramel. Chill until chocolate is set, about 30 minutes. Transfer the shortbread to a cutting board, carefully removing parchment paper. We found that chilling this recipe overnight made it easiest to cut. Cut into 24 small bars or any size you wish.
TIP: To easily cut into squares or bars….
Run a chef’s knife under hot water. Dry the knife then make the cuts while the knife is warm. You might need to do this a few times until all the bars are cut.