Whoopie Pies

1.

Preheat the oven to 350°F. Whisk together the flours, cocoa, baking soda, baking powder, and salt in a bowl or shake in a zipper-lock plastic bag until combined.

2.

In a large bowl or standing mixer, beat the oil and brown sugar with an electric mixer at medium-high speed until well combined. Add the egg whites and mix on low until well blended. Blend in the spinach puree and vanilla.

3.

Alternately add in the flour mixture and the buttermilk, beginning and ending with flour, mixing until smooth and just combined.

4.

Coat 2 large baking sheets with cooking spray. Place tablespoonfuls of batter about 2 inches apart onto baking sheets. Bake until the cakes are cooked through and spring back to the touch, 8 to 10 minutes. Transfer to a rack to cool completely.

5.

Make the filling. With an electric mixer on medium speed, beat together all the ingredients until smooth, about 3 minutes. Spread a rounded tablespoonful of filling on the flat sides of half of the cakes and top with the remaining cakes.