Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8-inch square baking dish.
In a blender or food processor, blend pumpkin, butter, eggs, and egg whites. Mix in the brown sugar, baking soda, salt, cinnamon, nutmeg, and cloves. Blend until smooth, and transfer to a medium bowl.
Mix yogurt and polenta into the pumpkin mixture. Pour into the prepared baking dish.
Bake until a toothpick inserted in the center comes out clean, about 45 minutes. Cool slightly before slicing.
