Brush chocolate sauce around the rims of glasses or mugs and place crushed Reese's Pieces on a small plate. Rotate glass upside-down on plate to coat rims in Reese's Pieces. Save any remaining Reese's Pieces for topping whipped cream.
Add chocolate to the bowl of a double boiler (or to a heat-proof bowl set over a pot of gently simmering water) and stir until melted. Whisk in cocoa powder and 1 cup whole milk. Stir until smooth.
Remove from heat and let mixture cool to room temperature (you can put it in the fridge to speed up process).
Pour remaining 1 cup milk, cool chocolate mixture, peanut butter, peanut butter cups and ice in blender and process until smooth.
Pour into prepared glasses or mugs and top with whipped cream and remaining crushed Reese's Pieces. Add on an extra peanut butter cup if desired.