1. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking soda, cream of tartar and salt; gradually add to the creamed mixture. Cover and refrigerate for 30 minutes or until easy to handle.
2. On a lightly floured surface, roll out dough to 1/4-in. thickness. For leprechauns, cut out dough with lightly floured 5-in. gingerbread boy cookie cutter. If desired, trim leprechaun's body for a thinner shape.
3. For each hat, cut a 1-1/2-in. square and a 1-3/4-in. x 1/4-in. brim from dough scraps. Place leprechauns 2 in. apart on ungreased baking sheets. Place hat squares and brims above heads, shaping gently to touch the heads.
4. Bake at 350° for 10-14 minutes or until edges are lightly browned. Cool for 1 minute before carefully removing to wire racks to cool completely.
5. Tint small amounts of Royal Icing red, yellow and black. Leave a small amount plain. Tint remaining icing green and orange. Frost leprechauns; decorate with shimmer dust and shamrock sprinkles if desired. For eyes, attach miniature chocolate chips with plain frosting. Yield: about 3 dozen.