1preheat oven to 350 and line a 9x13 inch pan with foil and spray well with cooking spray.
2for the cake layer: place stick of butter into a large bowl and microwave until the butter is softened (or just plan ahead and soften it at room temperature over time).
3add cake mix, egg and vanilla extract to the softened butter and mix with a spoon or your hands until well combined.
4it will be the consistency of very thick cookie dough and if its just not quite all combining, add 1 or 3 tablespoons of water as needed. press dough into prepared baking pan.
5for the marshmallow/peanut butter layer: place marshmallow creme, peanut butter, condensed milk and vanilla extract into a large bowl and mix until well combined. pour over the dough.
6for the white chocolate layer: sprinkle the white chocolate chips evenly over the top of the marshmallow/peanut butter layer.
7while the bars are baking, make the white chocolate vanilla butter cream frosting. add butter and white chocolate to a microwave safe bowl and melt in the microwave in 30-45 second increments, removing from the heat and stirring and microwaving until melted and smooth.
8don't scorth the white chocolate. add powdered sugar, cream and vanilla extract and stir until smooth and is in a semi pourable consistency. for thicker frosting, use less cream or powdered sugar. play around with ratios to your liking.
9after the bars are out of the oven, allow to cool for about 5-10 minutes and then drizzle the frosting over the top. you can also frost them in entirely rather then drizzling (you will have enough frosting to do so).
10allow to cool well before slicing and serving. expeclite this by placing in the fridge or freezer.