Simple sweet potato souffle

I invented this just to use up some leftover egg whites, but it was such a success I want to record the recipe for future use.  I didn't take a photo and six of us ate it all.  It isn't a true souffle that poufs, but it has a pleasant light texture and no added sugar.

2 medium sweet potatoes, cooked and peeled
2 tblsp butter
1/4 cup heavy cream
salt
5 egg whites
pinch of cream of tartar
1/4 cup chopped pecans

Oil a souffle dish or 8x8 pan and preheat oven to 400.  Process cooked sweet potatoes, butter and cream in a food processor until very smooth (a stick blender would probably also work).  Add salt to taste.  Beat egg whites with cream of tartar until stiff.  Mix a third of beaten egg whites into the sweet potato mixture, then fold the sweet potatoes gently into the egg whites until completely combined.  Pour into pan, smooth the top, and sprinkle with pecans.  Bake at 400 for 20-40 minutes, depending on thickness.