This is cobbled from multiple recipes.
1 cup unsalted butter, softened
3/4 c sugar
1 packed T lemon zest
1 1/2 t kosher salt
1 1/4 c shelled pistachios
1 1/2 c AP flour
1- 14 ounce can sweetened condensed milk
1 c packed/7 oz brown sugar
1/2 c heavy cream
1/2 c corn syrup
8 T melted butter
1/2 c whiskey
1/2 t salt
8 oz 70%-80% cacao bittersweet chocolate
2 T unsalted butter
Adjust oven rack to lower-middle position and heat oven to 350 degrees. Make foil sling for 13 by 9-inch baking pan by folding 2 long sheets of aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. In the bowl of a stand mixer, fitted with the paddle attachment, cream butter, sugar, lemon zest, and salt on high speed until fluffy, about 3 minutes.
Pulse the pistachios to a fine meal in a food processor. Add the ground nuts to the butter mixture and blend on low speed until incorporated. Still on low, add the flour, 1/2 cup at a time. When no flour streaks remain, increase speed to high and mix just until the dough holds together, about a minute.
Scrape the bough into the pan and compress with your hands. Bake until the edges brown and the top is dry but slightly sticky, about 15 minutes. The center should look underbaked. Let shortbread cool completely on a rack, still in pan.
Grind coriander in a spice mill, toast in a pan. Stir all ingredients together in large, heavy-bottomed saucepan. Cook over medium heat, stirring frequently, until mixture registers between 236 and 239 degrees (temperature will fluctuate), 16 to 20 minutes. Pour over crust and spread to even thickness (mixture will be very hot). Let cool completely, about 1 1/2 hours.
Melt chocolate and butter over low heat or double boiler, stirring frequently. When mostly melted, remove from heat and stir until smooth. Spread across butterscotch evenly, then sprinkle with flake salt. An offset spatula is needed to spread both chocolate and butterscotch. Sprinkle with flake salt.
Refrigerate until chocolate is firm, about an hour. Cut into 1″ squares and serve cold or at room temperature. They will keep in the freezer at least a month.