In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and salt. Beat till combined, scraping sides of bowl occasionally. Beat in egg yolk and vanilla till combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour with a wooden spoon. Divide dough in half. Form into 2 flattened rounds. Wrap dough in plastic wrap. Chill in the refrigerator about 1 hour or till easy to handle.

Roll dough between 2 sheets of waxed paper to a 1/4-inch thickness. Remove top layer of waxed paper. Using a floured 2-inch fluted or plain round cookie cutter, cut out dough. Carefully peel cutouts off the bottom layer of waxed paper. Place cutouts 1 inch apart on an ungreased cookie sheet.

Bake in a 350 degree F oven for 8 to 10 minutes or till edges are firm and bottoms are very lightly browned. Cool on cookie sheet for 2 minutes. Transfer cookies to a wire rack and let cool.

Up to 1 hour before serving, spread bottoms of half of the cookies with a rounded teaspoon of preserves. Top with remaining cookies, flat side down.

In a heavy small saucepan, melt bittersweet chocolate and the 1/2 teaspoon shortening over low heat, stirring constantly. Spoon melted chocolate mixture into a small, heavy self-sealing plastic bag; seal bag. Snip a tiny piece off a corner of the bag. Pipe on cookies to decorate as desired. Write the words that are important to you for the holidays such as peace, love and/or joy or decorate with stars, Christmas trees and/or hearts. Let stand till set. Repeat with white chocolate and the 1/2 teaspoon shortening. Spoon melted chocolate mixture into a small, heavy self-sealing plastic bag; seal bag. Snip a tiny piece off a corner of the bag. Decorate as directed above or as desired. Makes 14 sandwich cookies.