Preheat the oven to 350 degrees F.
Line an 8-inch square baking dish with parchment paper, allowing it to hang over the sides. Coat the paper with cooking spray.
To a large bowl, using a hand mixer, mix together the melted butter with the sugar and light brown sugar until smooth, about 3 minutes. Add the eggs and vanilla and mix on low, until combined.
Stir in the flour and salt until just incorporated - no need to overmix. Stir in ½ cup of the assorted Halloween candy.
Pour the cookie batter into the prepared pan, smoothing the top, then sprinkle the remaining ½ cup of Halloween candy over the top of the batter.
Bake 30-35 minutes until done.
Let the pan cool completely then remove the cookie bars, using the parchment paper overhang, cut into 12-16 squares. Top with flaky sea salt and serve.