Instructions

Make the whoopie pies:

1. Preheat oven to 375 degrees.  Line two baking sheets with parchment paper.
2. Combine flour, baking powder and salt.  Set aside.  Cream the butter, shortening, and brown sugar until light and fluffy (approx 3-4 min).  Add eggs and buttermilk and beat until combined.  (It will look curdled, that is OK.)
3. In a 1 cup measuring cup combine milk, baking soda, and vinegar. With your mixer on low pour in half of the flour mixture, then the milk, then the rest of the flour.  Mix until just combined.  Stir in vanilla.
4. Use a small cookie scoop to drop dough onto baking sheets.  Leave about 2 inches between the cookies.  Bake for about 8-10 minutes or until they are set, and just starting to brown around the edges.  Let cool for a few minutes, then cool on a wire rack.

Make the frosting:

1. Cream butter and brown sugar on medium-high speed for about two minutes, or until fluffy.
   Add vanilla and milk. Mix until incorporated.
2. Next, add flour and mix on low speed.
3. Add powdered sugar, mixing on low speed, until just combined. Mix on medium for another minute or two.
4. If the frosting is too thick at this point, add another tablespoon of milk. If it’s too runny, add 1/4 cup flour or powdered sugar.