Prep. First, heat the oven to 350°F (175°C). Then, cover a 9 x 9 or 9 x 13 pan with aluminum foil, making sure there is extra foil on the sides to make handles. Remember to spray the foil with cooking spray.
Mix wet and dry ingredients separately. In a large bowl, beat the melted butter and brown sugar with an electric mixer until well combined. Then, add the eggs and vanilla extract and beat until smooth. In a separate bowl, combine the flour and salt. Gradually add the flour mixture to the butter mixture, stirring until just combined.
Finish batter. Add the Heath bar bits and stir gently.
Bake. Pour the batter into the prepared pan and spread it evenly with a spatula. Then, bake for 35 minutes in a 9 x 9 pan or 25 minutes in a 9 x 13 pan, or until the blondies seem firm but slightly undercooked (they will keep cooking as they cool). Let the blondies cool entirely in the pan. After they cool, use the foil handles to take the blondies out of the pan and cut them into squares.