Melt chocolate baking bar in double boiler and set aside. Mix flour and salt together and set aside. In large bowl, beat butter and sugar until smooth. Add melted chocolate and vanilla. Add flour mixture to chocolate mixture and mix just until blended.
Wrap dough in plastic wrap and chill for at least 1 hour. Heat oven to 350°F. Line baking sheet with parchment paper. On a lightly floured board, roll dough to 3/8" thick and cut with cookie cutters. If the dough is very soft, chill again for at least 30 minutes.
Bake for 7-9 minutes. Cool completely in pan.