- Lightly grease pie pan and set aside. Cut butter into ½-inch cubes. Add butter to flour and work mixture with hands until it resembles coarse meal. Add water and mix together to form solid dough, careful not to overwork. On a floured workspace, roll dough to 1/8-inch thickness.
- Place dough in prepared pie pan. Trim away excess dough. Score dough on bottom of pie pan with fork.
- Preheat oven to 350 degrees. Peel sweet potatoes and chop into ½-inch dice. Set aside.
- Pour water into 3-quart stockpot and bring to boil over medium-high heat. Suspend strainer basket over pot and add sweet potatoes. Cover and steam until fork tender, about 20 minutes. Remove potatoes and let cool to room temperature.
- Once cool, mash sweet potatoes into smooth puree with fork or potato masher. Add butter, lemon juice, cinnamon, and salt to ¼ cup puree. Mix thoroughly.
- In small bowl, beat egg yolks lightly with whisk, about 30 seconds. Add sugar and beat until creamy and lemon yellow in color, about 1½ minutes. Add egg mixture to sweet potato mixture and stir until combined and filling is bright orange in color. Slowly add flour and mix until incorporated. Add buttermilk and stir until filling is smooth and even.
- In clean, dry bowl, whisk egg whites into soft peaks. Using rubber spatula or wooden spoon, gently fold egg whites into sweet potato mixture.
- Pour mixture into piecrust. Bake on middle rack of oven until center is firm and set, 40–45 minutes. Remove from oven and let cool completely. Serve at room temperature or cold, topped with generous dollop of whipped cream.