Adjust oven rack to middle position; heat oven to 350° F. Spray an 8-inch pan with cooking spray or grease lightly with oil. Cut an 8 x14-inch wide strip of foil and line pan foil with the 2 long sides extending over the top of pan. (You will use this later to easily remove bars from pan.) Spray or rub with oil to coat foil.

Heat butter in a medium large saucepan till just melted. Remove from heat and allow cool slightly for a few minutes. Whisk in sugar, mixing until well combined. Add egg and vanilla, stir to incorporate.

Add flour, baking powder, and salt together to mixture in pot. Using a rubber spatula stir just until combined; do not overmix.

Bake until top is shiny, cracked, and light golden brown, 18 to 22 minutes; do not overbake. Set aside to cool for 10 minutes. Set broiler to high with rack set 6-8inches from broiler.

Top blondies with chocolate chips (or lay candy bars across top to cover), then sprinkle with marshmallows. Place under the broiler for approximately 30 seconds to one minute or until marshmallows start to brown. Watch very carefully as marshmallow will quickly go from golden brown to burned.

Remove from oven and cool for 10 minutes then add Hershey bar pieces, randomly inserting them across top. I like to make it look like they’re “peeking” out from the marshmallows.

Set aside to cool completely. When ready to serve, lightly oil a sharp knife and cut into 2-inch squares.