Heart Healthy Sweet Potato Pie

Ingredients

• 3 Tbsp unsalted butter
• 1/2 cup sugar
• 1/4 tsp salt
• 1 large egg yolk
• 3 Tbsp fresh lemon juice
• 1 tsp grated lemon zest, plus additional for garnish
• 1/2 tsp ground cinnamon
• 3 large sweet potatoes, cooked, peeled and mashed
• 1 cup fat free evaporated milk
• 3 large egg whites
• 1 (9-inch each) prepared pie shell, unbaked
• 1/4 cup fat free whipped topping (optional)