For the crust:

1. Position rack to lower middle and preheat oven to 350 degrees F.  Line a 9×13 inch baking pan with a parchment sling. (I like to lightly spray the pan with non-stick spray before setting in the parchment.)
2. In a medium bowl combine flour, sugar, and salt. Add in melted butter and mix with a spatula until all flour is combined. Crumble mixture over bottom of pan and pat down with hands and smooth out. Prick dough with a fork every couple of inches to prevent it puffing during baking. Bake for 25-30 minutes. Let crust cool for 15 minutes before adding filling.

For the filling:

1. While crust is cooking/cooling, make the filling. In a large, heavy saucepan, melt butter and brown sugar on low heat. Whisk in cream, dark corn syrup, condensed milk, and salt and turn heat to medium. Continue to stir until mixture reaches between 236 and 239 (just the beginning of the soft ball stage). This will take 15-20 minutes.
2. Use a heat proof spatula to scrape the bottom of the pan to ensure mixture doesn’t burn. Once the filling has reached the soft ball stage, remove from heat and pour over slightly cooled crust. Spread mixture evenly with an offset spatula and let cool completely, approximately 90 minutes.

For the chocolate topping:

1. Roughly chop 12 ounces of baking chocolate. Finely grate the remaining 4 ounces.
2. In a heat proof bowl, set over slightly boiling water, melt the chopped chocolate. Slowly melt the chocolate, stirring with a spatula. When chocolate has melted, turn off heat and add the grated chocolate.  Stir to combine.
3. Pour over set filling and spread with an offset spatula. Pop into the freezer for 5-10 minutes to help set the chocolate.

To serve: Remove the parchment sling and using a sharp knife, cut the shortbread in half width-wise. Repeat with two smaller pieces of shortbread (now you’ll have four). Cut each slab into four or five bars. Can be stored, covered, at room temperature for 3 days.