1. Preheat oven to 300 degrees F (150 degrees C).
2. In heatproof bowl over saucepan of hot (not boiling) water, melt chocolate, stirring occasionally. Remove from heat and let cool.
3. Meanwhile, in bowl, beat butter with sugar until fluffy.
4. Stir in chocolate and vanilla.
5. Add flour and salt; stir until blended. Turn out onto lightly floured work surface.
6. Form into 4 bricks, each about 1-inch (2.5 m) high and 1-1/2 inches (4 m) wide. Cover with plastic wrap and refrigerate for 2 hours or until firm. (Make-ahead: Refrigerate for up to 12 hours.)
7. Line rimless baking sheets with parchment paper.
8. Cut dough into 1/4-inch (5 mm) thick slices and place on prepared pans. Prick each twice with fork.
9. Refrigerate for 1 hour or until chilled. (Make-ahead: Freeze on baking sheets until firm; layer between waxed paper in airtight container and freeze for up to one month. Bake from frozen, adding 5 minutes to baking time.)
10. Bake in centre of oven for about 30 minutes or until firm to the touch.
11. Let cool on pans on racks for 2 minutes.
12. Transfer to racks and let cool completely.
13. Yield: 56 cookies
14. Chocolate Ginger Shortbread: Add 1 cup (250 mL) chopped crystallized ginger to dough before shaping.
15. Dark Mountain Shortbread Mounds: Add 1 cup (250 mL) finely chopped nougat bar (Toblerone) to dough. Drop by rounded tablespoonfuls (15 mL) onto pans’ form into mounds and press 1 cm chunk of bar into each mound. Bake and cool as above. Sieve icing sugar over each.