In a large saucepan, combine the sugar, cornstarch and salt. Stir in buttermilk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in the raisins, lemon juice, butter and vanilla until butter is melted. Cool to room temperature without stirring. Pour hot filling into crust.
For meringue, in a large bowl, beat egg whites 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 the meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving. Store leftovers in the refrigerator.