In a medium bowl, combine the melted butter, creamy peanut butter, and powdered sugar. Use a hand mixer to beat the mixture until smooth and fluffy. Set aside.

Scoop enough vanilla ice cream into your blender until it fills about three-quarters of the blender. Add milk until it reaches halfway up the ice cream. Blend on high until the mixture becomes silky and smooth. If the shake is too thick, add a little more milk (about a tablespoon at a time) and blend again until you reach your desired consistency.

In each serving glass, spoon a generous scoop of both the hot fudge sauce and the peanut butter sauce into the bottom. Pour the vanilla shake into the glass until halfway full. Add another spoonful of each sauce. Continue filling the glass with more shake, leaving a little room at the top for finishing touches.

Use a chopstick or butter knife to gently swirl the sauces through the shake. Move in a circular or figure-eight motion for beautiful, marbled ribbons of chocolate and peanut butter.

Top each shake with a drizzle of extra sauce and, if you like, a mini peanut butter cup perched right on top. Serve immediately with a wide straw or long spoon.