Preheat oven to 350°F. For shells, in mixer bowl, cream butter and sugar just until blended. Add flour and salt; stir until mixture appears crumbly (mixture will be dry). In small bowl, combine egg yolk and vanilla; drizzle over flour mixture and mix until evenly moist (mixture will be crumbly). Divide mixture between pans. Press dough evenly over bottom and up sides of pans, trimming off excess. Bake 16-20 minutes or until golden brown. Cool 15 minutes in pans; carefully remove shells from pans. Cool completely on cooling grid.