How to Make It
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.