Beat butter at medium speed of an electric mixer until creamy; gradually add sugar, beating until fluffy. Add egg yolk and vanilla; beat well.
Combine flour and salt; add to butter mixture, beating well. Stir in 1 cup pecans. Cover and chill dough at least 30 minutes.
Lightly beat egg white. Shape dough into 1" balls; dip each ball in egg white, and roll in remaining 1 cup pecans. Place balls 1" apart on greased cookie sheets. Press thumb gently into center of each ball, leaving an indention; fill with preserves.
Bake at 350° for 17 to 18 minutes or until lightly browned. Cool 1 minute on cookie sheets; remove to wire racks, and cool completely. Drizzle melted chocolate over cooled cookies, using a fork or spoon.