Preheat the oven to 350ºF.
Line an 8-inch square baking pan with parchment paper, allowing it to hang over the sides. Coat the paper with cooking spray.
In a stand mixer fitted with the whisk attachment, whisk together the melted butter with the sugar and light brown sugar until smooth, about 2 minutes. Add the eggs and vanilla and whisk until combined.
Remove the bowl from the stand mixer and stir in the flour and salt just until incorporated. Stir in 1/2 cup of the assorted Halloween candy.
Pour the batter into the prepared pan, smoothing the top, then sprinkle the remaining 1/2 cup of Halloween candy over the top of the batter.
Bake for 30 to 35 minutes until a toothpick inserted comes out clean.
Let the pan cool completely then remove the blondies using the parchment paper overhang, cut into 16 squares and serve.