Step 1: Preheat your oven to 350°F (175°C) and line a 9×13 inch baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.
Step 2: In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until the mixture is light and fluffy, about 3-4 minutes.
Step 3: Beat in the eggs one at a time, then stir in the vanilla extract until well combined.
Step 4: In a separate medium bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Step 5: Gently fold in 2 cups of the chopped trick or treat cookie bars candy mix, saving the remaining 1/2 cup for the top.
Step 6: Press the dough evenly into the prepared baking pan. Sprinkle the remaining candy pieces and flaky sea salt over the top, pressing them down lightly into the dough.
Step 7: Bake for 22-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out mostly clean. Do not overbake; they will firm up as they cool.
Step 8: Allow the trick or treat cookie bars to cool completely in the pan before lifting them out by the parchment paper and slicing into squares.