Preheate oven to 350 F (175 C). Place baking sheet on the bottom oven rack to warm it up.
Using blender or food processor, mix sweet potato with butter. Add salt, milk and egg yolks and blend until smooth. Add additional ingredients for the sweet or savory savory sweet potato souffle.
Whisk egg whites with lemon juice until soft peeks form. Gently fold into sweet potato mixture using rubber spatula or wooden spoon.
Spoon mixture to the top of buttered remekins. Level off the tops with a knife.
Place on the hot baking sheet and bake for about 35 minutes in the oven (with fan turned off).
Serve immediately, garnished with pecans and maple syrup or chopped chives (optional).