In a small saucepan, combine cornstarch and water until smooth. Stir in syrup and cream. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to medium; cook, stirring constantly, 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in butter. Pour into pastry shell. In a large bowl, beat egg whites, vanilla and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form and sugar is dissolved. Spread evenly over hot filling, sealing edges to crust. Bake at 350° for 12-15 minutes or until meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving.