Preheat oven to 350 degrees.

Grease the bottom and sides of a 9-inch ceramic or glass pie plate with butter. Set aside.

Melt 2 tbsp butter in skillet over medium heat and add the leeks. Saute until tender and fragrant, then set aside.

Beat the egg whites until stiff peaks form and hold their shape. Set aside. I

n a medium sized bowl, mix the sweet potato puree, butter, ricotta, egg yolks, and as much salt and pepper as you like. Stir in the cooked leeks, mixing well. Fold in the egg whites, very gently. Spoon into the prepared pie dish and bake for one hour, until puffed and golden.

Enjoy!