Pistachio Millionaire's Shortbread with Coriander Butterscotch

1 cup (two sticks) unsalted butter, softened

3/4 cup sugar

1 1/2 teaspoon kosher salt

Zest of one lemon

1 1/4 cup (8 ounces) shelled pistachio nuts

1 1/2 cup all-purpose flour

For coriander butterscotch

1/2 cup (1 stick) unsalted butter

1 1/2 cup raw (turbinado) sugar

1/2 cup heavy cream

1/2 cup Irish whiskey

2 tablespoons coriander, toasted and freshly ground, plus more for dusting

2 teaspoons kosher salt

For chocolate topping

8 ounces dark chocolate, about 70% cacao, chopped fine

2 tablespoons unsalted butter