1. Preheat oven to 350 degrees F. Coat two baking sheets with cooking spray or line with parchment paper and set aside.
2. In a large mixing bowl, use an electric mixer on medium speed to mix butter and brown sugar until well blended, about 3 minutes. Add eggs, one at a time, mixing until smooth after each egg. Mix in vanilla extract.
3. In a separate bowl, stir together flour, cocoa powder, baking soda and salt. Add half the flour mixture to mixing bowl and mix on medium speed until combined. Add half the buttermilk and mix again on medium until smooth. Repeat with remaining flour and buttermilk, and mix until smooth. Batter will be thick and slightly springy.
4. Drop 2-Tbsp. portions of batter onto prepared baking sheets, leaving 2 inches between each portion to allow for spreading. Bake 11 to 13 minutes, until puffed and set but still soft when touched lightly with fingertips. Let cakes cook for 3 minutes on baking sheets before transferring them to wire racks to finish cooling, about 15 to 20 minutes. Repeat with remaining batter.
5. Prepare filling while cakes are baking and cooling. In a large mixing bowl, use an electric mixer on medium speed to mix crea, cheese, peanut butter, vanilla and salt until creamy. Add half the confectioners' sugar and mix on low first to combine, then on high until smooth. Add remaining confectioners' sugar and milk and blend on low, then on medium until light and fluffy, about 3 to 4 minutes.
6. Assemble whoopie pies. Spoon filling onto the flat sides of half the cakes, dividing it evenly. Top with remaining cakes, flat side against the filling, rounded side up. Serve immediately, or wrap and store at room temperature for up to two days, or in freezer for up to two months.
Source: Hannaford fresh Magazine, July - August 2008