Spray an 8x8" baking pan with non-stick cooking spray.
In a large saucepan, melt marshmallows with butter and corn syrup over medium-high heat, stirring until completely smooth. Add in vanilla pudding mix, vanilla, and pumpkin pie spice and stir well. Pour over cereal in bowl and mix until completely incorporated, then press into prepared pan. Let set up for about 20 minutes and prepare frosting while waiting.
For the frosting, cream butter in a mixing bowl with an electric mixer on medium speed until butter is soft and fluffy. Gradually beat in confectioners' sugar, about 1/2 cup at a time; beat in vanilla extract. Gently fold the marshmallow creme into the frosting until thoroughly incorporated. Spread frosting on top of crispy treats and cut into bars.