Make your Hot Fudge Sauce (if using homemade) and Peanut Butter Sauce. Combine the melted butter with the creamy peanut butter and 1/2 cup powdered sugar. Use a hand mixer to mix until smooth
Scoop ice cream into a blender. I fill my blender up 3/4 full with vanilla ice cream and it makes about 4 tall shakes. Add in milk halfway up the ice cream. Blend until smooth. You may need to add a little more milk if it is too thick.
Scoop a spoonful each of the hot fudge sauce and peanut butter sauce into each cup. Fill the cup halfway with the vanilla shake, then spoon another helping each of the hot fudge and peanut butter sauce. Fill the cup to the top and top with drizzles of the sauces. Use a long stick (chop stick, butter knife, etc) to swirl the sauces immediately. The sauces will thicken as the ice cream cools them off, but will remain soft enough that you don't end up with hard bites in your shake.
(Optional) place a Reese's Peanut Butter Cup on the top of each shake.