- 3/4 cup (1 1/2 sticks) unsalted butter, plus more for the pan
- 1 1/2 cups all-purpose flour, spooned and leveled
- 1/2 teaspoon baking powder
- 1 teaspoon fine salt
- 3/4 cup packed light brown sugar
- 1 large egg
- 1/2 teaspoon pure vanilla extract
- 6 ounces chocolate covered toffee bar, broken into small pieces (1 1/4 cups)
- Heat oven to 325° F. Butter an 8-by-8-inch baking dish and line with parchment, leaving an overhang on the two long sides; butter the parchment. Whisk together the flour, baking powder, and ½ teaspoon of the salt in a medium bowl; set aside.
- In a separate bowl, beat the butter and sugar on medium-high until light and fluffy, 3 to 4 minutes. Beat in the egg and vanilla until combined. Reduce speed to low and gradually add the flour mixture, mixing just until combined (do not overmix).
- Spread the batter in the prepared pan and bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes.
- Remove the pan from the oven and scatter the toffee pieces on the top. Return to the oven and bake until the toffee is melted, 1 to 2 minutes more. Sprinkle with the remaining ½ teaspoon of salt. Cool completely in the pan.
- Holding the paper overhang, lift the bars out of the pan and transfer to a cutting board. Cut into 16 squares.
- Storage suggestion: Keep the bars at room temperature, between sheets of wax paper or parchment in an airtight container, for about 5 days.