Let butter sit on the counter until soft. Chop candy bars into small pieces. Line a cookie sheet with parchment paper or silicone liner, or spray with cooking spray. Preheat oven to 350°.

Combine butter and sugars in a large bowl. Beat for 2-3 minutes with an electric mixer at medium speed, or until very creamy. Add the eggs and vanilla, beat until smooth.

Add the flour, cornstarch, baking soda, baking powder, and salt. Stir until barely combined, then carefully stir two cups of the candy pieces.

Use a medium cookie scoop to scoop about two tablespoons of dough. Roll into balls.

Place cookie dough balls about 3" apart on prepared cookie sheets. Bake at 350 degrees for about 10-11 minutes or until edges are golden brown and middles are barely set.

Immediately press a few chocolate candies on top of each cookie. Let cool for 3-4 minutes, then transfer to a cooling rack to cool completely.