Melt the Marshmallow Mixture: Combine the melted butter, light brown sugar, mini marshmallows, and vanilla extract in a medium saucepan. Cook over medium heat, stirring constantly, until completely melted, smooth, and glossy. Remove from the heat immediately.
Coat the Popcorn: Pour the marshmallow mixture over the popped popcorn in a large bowl. Use a greased nonstick spatula to fold and toss gently until every kernel is thoroughly coated.
Form the Skulls: Line a large baking sheet with parchment paper. Spray a 1-cup dry measuring cup and your hands lightly with nonstick cooking spray. While the popcorn mixture is still warm, scoop 1 cup of popcorn, pack it firmly, unmold it, and shape it into a skull around a popsicle stick with your hands. Place on the prepared sheet and repeat to make 8 skull pops.
Decorate: Use a small, clean food-safe paintbrush to apply a dab of lightcorn syrup to the back of your candies, then press them firmly onto the popcornskulls to create eyes, noses, teeth, and decorative patterns.
Set: Let the decorated skulls stand at room temperature for about 30 minutes until the corn syrup dries and the decorations are secure before serving.