Mist a large bowl with cooking spray. Line a baking sheet with parchment. Warm oil in a large pot over high heat. Add popcorn, partially cover and cook, shaking pot. When corn stops popping, remove from heat. Transfer popcorn to bowl.
Add butter, sugar and marshmallows to pan; cook over medium heat, stirring, until melted and smooth. Continue to cook, stirring, for about 5 minutes. Pour over popcorn; stir to coat. Let cool slightly.
Mist hands with cooking spray. Form popcorn into 8 skulls around craft sticks. Set on parchment. With clean hands, press M&M's and candy corn into skulls to make eyes and teeth.