Pour the milk in a small saucepan, and over a medium heat, bring to a simmer.
While the milk is heating, whisk the eggs, egg yolks, sugar, lemon extract, and salt together in a large bowl, and gradually add the hot milk to the egg mixture, stirring constantly. Pour the custard into the prepared pie crust.
Bake, about 25-35 minutes, or until the center of the custard is set, but still jiggles a bit, like gelatin. Remove from oven and let cool completely on a cooling rack, then refrigerate. Once the pie is cold, begin the lemon curd topping.
For lemon curd topping, combine ½ cup granulated sugar, cornstarch, and salt, and water in a small saucepan, stirring until smooth. Bring mixture to a boil over medium-high heat. Reduce heat, and add the remaining 2 tablespoons sugar. Cook and stir until thickened and bubbly, about 2 minutes. Remove sugar mixture from heat, and stir in butter, lemon zest, and food coloring, if using until butter is melted. Gently stir in lemon juice. Let lemon curd cool to room temperature, about ½ hour. Pour lemon curd topping over cooled custard pie. Refrigerate overnight.