Halloween M&M Popcorn Cake

Butter the inside of a 13x9 inch baking pan. Combine the butter, oil and mini marshmallows in a large saucepan over low heat. Cook, stirring constantly, until the mixture is melted and smooth. Remove the pan from the heat and stir in the popcorn until evenly coated. Gently stir in the M&M's. Spread the popcorn mixture evenly in the prepared pan. Cool completely before cutting into squares and serving.