I had the opportunity to fill and decorate one myself on a recent trip to Dunkin' Donuts headquarters (disclosure: travel and accommodations provided by Dunkin' Donuts). They normally cost $1.49.
The creme filling sets it apart with a full peanut butter flavor but with a whipped cream texture (whoever piped in the creme did an excellent job!). There's just a bit of saltiness to it. Combined with the chocolate frosting, there's an nice chocolate-peanut butter combo; although I wonder if it could be enhanced by the chocolate yeast donuts they had a while back.
Nutritional Info - Dunkin' Donuts Reese's Peanut Butter Square
Calories - 370 (from Fat - 170)
Fat - 19g (Saturated Fat - 7g)
Sodium - 400mg
Carbs - 47g (Sugar - 26g)
Protein - 5g