Preheat oven to 350˚F.
To Make CRUST: Mix butter, brown sugar and vanilla using an electric mixer. Gradually blend in flour and corn starch. Press into bottom and up sides of a 9-inch ungreased spring form pan.
Bake at 350˚F for 20 minutes or until lightly browned. Remove from oven. Increase oven temperature to 450˚F.
To Make FILLING: Whisk sugar, corn starch, lemon juice and eggs in a heavy saucepan over medium heat.
Cook, stirring constantly for 8 to 10 minutes, or until thick . Stir in sour cream and butter. Immediately pour over hot crust.
To Make MERINGUE: Beat egg whites and cream of tartar in large bowl with an electric mixer until soft peaks form. Gradually beat in sugar until stiff peaks form.
Mound meringue evenly over warm filling, spreading to edges of pan.
Bake at 450˚F for 5 to 6 minutes until lightly browned. Cool at least 30 minutes before serving. Store leftovers in refrigerator.