Bring 1 cup buttermilk to a boil. Mix together the sugar, cornstarch and 1 /2 cup buttermilk. Add to the boiling buttermilk and cook while stirring, until of a smooth cream consistency. Beat the egg yolks and add the salt, grated lemon rind and the lemon juice. Pour into the hot cream. Add the butter. Cook for 2 minutes, stirring vigorously. Cool. Pour into a baked pie shell. Top with 2 egg whites, stiffly beaten with the 1 /4 cup sugar. Brown in a 350°F oven for approximately 20 minutes. Note: Another example of the thriftiness and ingenuity of the early Acadians.