- 1 cup (2 sticks) unsalted butter, softened, plus more for pan
- 2 cups all-purpose flour
- 1 1/4 teaspoons coarse salt
- 3/4 cup packed light-brown sugar
- 1 cup finely chopped chocolate-covered toffee bars plus 1/2 cup coarsely chopped chocolate-covered toffee bars (about 10 ounces total)
- 1/4 cup heavy cream
- Preheat oven to 300 degrees, with rack in upper third. Butter an 8-inch square baking pan. Whisk to combine flour and salt into a bowl.
- In a separate bowl, beat butter with an electric mixer on medium-high until creamy, 2 to 3 minutes. Gradually add brown sugar; beat until fluffy, about 2 minutes more. Reduce speed to low. Add flour mixture; mix just until combined. Using plastic wrap, press dough into prepared pan. With plastic on dough, chill 45 minutes.
- Pierce dough all over with a wooden skewer. Bake, rotating pan halfway through, until golden brown and center is firm, about 70 minutes. Transfer to a wire rack; let cool 10 minutes.
- Meanwhile, heat finely chopped toffee bars and cream in a saucepan, stirring constantly, over medium-low heat until smooth. Pour over shortbread in pan; sprinkle with coarsely chopped toffee. Let cool slightly. Cut into squares. Let cool completely.
Cookies can be stored in an airtight container at room temperature up to 1 week.