DIRECTIONS:

1. Preheat oven to 350. Whisk together flours, cocoa, baking soda and powder, and salt in a bowl.
2. In a standing mixer, beat the oil and brown sugar with an electric mixer at medium speed until well combined. Add egg whites and mix on low until well blended. Blend in spinach puree and vanilla.
3. Alternately add in the flour mixture and the buttermilk, beginning and ending with flour.
4. Coat 2 large baking sheets with cooking spray. Place tablespoon-fuls of batter about 2 inches apart onto baking sheets. Bake until cakes are cooked through and spring back to the touch, 8-10 mins.
Make the filling. With an electric mixer, beat together all the ingredients until smooth. Spread a rounded tablespoonful of filling on the flat sides of half the cakes, and top with the remaining cakes.