For the shortbread base:
1½ cups all-purpose flour
½ tsp. salt
¼ tsp. ground cinnamon
2 sticks (1 cup) unsalted butter, at room temperature
½ cup brown sugar, packed
¼ cup granulated sugar
1 tsp. vanilla extract
3 oz. bittersweet or semisweet chocolate, finely chopped
For the topping:
6 oz. bittersweet or semisweet chocolate, finely chopped
1½ cups toffee bits
Preheat the oven to 375˚ F. Line a 9 x 13″ baking dish with foil and spray lightly with cooking spray. To make the shortbread, combine the flour, salt and cinnamon in a small bowl; stir to blend and set aside. In the bowl of an electric mixer, combine the butter and sugars and beat on medium-high speed until light and fluffy, about 3 minutes. Blend in the vanilla. With the mixer on low speed, beat in the dry ingredients just until incorporated. Stir in the chopped chocolate until evenly distributed.
Transfer the dough to the prepared baking dish and press into an even layer covering the bottom of the pan. Bake for 18-20 minutes, or until the shortbread is bubbling. Remove the pan from the oven and transfer to a wire rack.
Sprinkle the 6 ounces of chopped chocolate evenly over the top of the cookie base. Cover the pan with foil and let sit for 5 minutes. Remove the foil and use an offset spatula to spread the melted chocolate evenly over the crust in a thin layer. Immediately sprinkle the toffee bits over the melted chocolate and press down gently to adhere. Let the bars cool to room temperature. Once the pan has cooled, you may speed chilling by transferring to the refrigerator or freezer. When the topping is set, remove from the pan, transfer to a cutting board and slice into bars.