Combine all the ingredients, up to the hot fudge sauce, in a blender. Blend just until mixed, adding more milk or ice cream to achieve the desired consistency.

Place 1 tablespoon of hot fudge sauce in the bottom of two tall glasses; pour half of the milkshake into each glass. Add another tablespoon of hot fudge sauce to each glass. Divide remaining milkshake between the two glasses and add the final two tablespoons of fudge sauce to each glass.

Garnish shakes with whipped cream and extra peanut butter cups, if desired.