In a mixing bowl, cream together the melted butter and sugars using a wooden spoon, hand mixer or stand mixer. Add the egg and egg yolk, and vanilla, then mix to combine.
Next add the all-purpose flour, baking soda, baking powder and salt. Stir to mostly combine. Add the candy-coated pieces (M&M's, Reese's, etc...). Stir combine but do not overmix.
Preheat the oven to 350°F, and line an 8x8 metal baking pan with parchment paper, or spray with non-stick baking spray.
Deposit the cookie dough into the prepared baking pan and gently press until it forms an even layer.
Bake cookie bars for 24-27min or until they are light golden brown around the edges, and the center looks soft, but set.
While the cookie bars are baking, prep the Halloween candy - unwrap and set aside. Cut or break larger pieces into smaller pieces, if desired.
Remove the cookie bars from the oven and immediately press the Halloween candy into the top of the cookie bars while still hot.Allow cookie bars to cool completely before removing from the pan and cutting, Store cooled cookie bars in an airtight container for 3-5 days at room temperature. See Recipe Notes for tips on freezing.