In a large bowl or bowl of a stand mixer, combine the melted butter, granulated sugar, brown sugar and vanilla. Mix on medium speed for until combined. Add the eggs and mix on low until just incorporated.
Add the flour, cornstarch, cocoa powder, baking soda and salt. Mix on low until just combined. Fold in the chopped Halloween Oreos, semisweet chocolate chips, M&Ms, sprinkles and chopped pretzels (or any other mix ins you'd like) until evenly distributed.
Line a baking sheet with parchment paper. Scoop the dough into 20 even balls, about 2½ to 3 tablespoons each. Refrigerate the scooped cookie dough for at least 30 minutes.
Preheat the oven to 350°F. Place 4-5 of the chilled cookie dough balls about 2 inches apart on a parchment lined baking sheet. Bake for 10 to 12 minutes, or until the edges are just set. The centers should still look slightly soft.
Immediately after removing the cookies from the oven, use a round cookie cutter or glass to gently scoot each cookie into a round shape, if desired. While the cookies are still warm, press additional Oreo pieces, chocolate chips, pretzels, candy corn, candy eyeballs and assorted Halloween candies into the tops. Let the cookies cool on the baking sheet for about 3 minutes, then transfer them to a wire rack to cool completely. Serve and enjoy!