Preheat your oven to 400 degrees F. Pierce the sweet potatoes all over with a fork, then wrap them in foil. Bake in your oven for an 1 1/2 hours. Remove from the oven and let stand to allow cooling completely. Unwrap and slice the baked potatoes in half, return the pieces to the oven. Reduce the temperature to 275 degrees F, and bake for about 10 minutes more. Scoop the flesh out; combine with milk and process in a blender until smooth.

Transfer the blended mixture into a heavy pan over medium heat. Stir in the cornmeal; bring to a simmering boil, while whisking briskly. Lower the heat to medium-low; continue to cook for 10 minutes more to obtain a thick consistency. Turn off the heat and stir in the butter, Stevia, salt, and spices. Mix to blend well. Let stand to cool.

Beat the eggs in a small bowl; add 1/2 cup cornmeal mixture and beat again. Pour this mixture into the rest of the batter, and beat together well. Stir in the yogurt, then pour the batter into a greased baking dish; sprinkle the top with walnuts. Bake at 270 degrees F for 2 hours.

Serve warm. Top with ice cream or whipped cream if desired (2 tablespoons or 1/2 cup adds 1 serving of fat)