In a medium bowl, whisk together the flour, baking powder, and salt. In a separate bowl, whisk together the butter, egg whites, and vanilla. Stir in the Greek yoghurt, mixing until no large lumps remain. Mix in the honey and stevia. Alternate between adding the flour mixture and milk, beginning and ending with the flour mixture, and stirring just until incorporated. (For best results, combine the flour mixture in 4 equal parts.) Gently fold in 1 cup of blueberries.