Lightly spray a baking sheet, and preheat oven to 350 degrees. Combine butter, cream cheese, and sugars stirring with a whisk until smooth and creamy. Add vanilla and yogurt and stir to blend. In a seperate bowl combine the flours, cocoa, soda, and salt stirring with a whisk. Add the dry ingredients to the butter mixture, and stir until smooth. Place small scoops of the batter on the prepared sheet. Bake for 8 minutes, no longer. Remove from the oven and allow the cookies to cool. Dust with powdered sugar (if desired.) This makes approximately 36 cookies.