In a small microwave-safe bowl, microwave the chocolate on 100% power for 1 minute. Stir well.
Continue microwaving on 5-% power, stirring at 30 second intervals.
Stop microwaving before the chocolate completely melts and let residual heat finish the job.
(Alternatively, in small heavy saucepan, melt the chcolate over lowest beat, stirring frequently; be very careful not to burn. Immediately remove from heat.let cool to warm.
In a large bowl, with electric mixer on medium speed; beat together the butter and sugar until well blended lightened. Add the egg, vanilla and salt and beat until well blended and smooth.
Reduce speed to low and beat in powdered sugar and 1/2 cup of the flour, then the chocolate, until well blended. Beat or stir in remaining 1/2 cup of flour until evenly incorporated.
Divide dough in half.
Place each portion between large sheets of wax paper.
Roll out the portions 1/8" thick, check the underside of the dough and smooth out any wrinkles. Stack the rolled portions( paper still attached) on a baking sheet. Refrigerate for 45 minutes or until chilled and firm, or freeze.for about 20 minutes to speed chilling.
Preheat oven to 350*. Grease several baking sheets or coat with nonstick spray.
Working with one portion at a time and leaving remaining dough chilled, gently peel away, then pat one sheet of waxed paper back into plate.
Flip the dough over, then peel off and discard the second sheet. using a 2 1/2"-3" fluted round cookie cutter or any shape desired. cut out the cookies(if at any point the dough softens too much to handle, transfer the paper and and cookies to a baking sheet and refrigerate until firm again. using a spatula, transfer cookies to baking sheets, spacing 1 1/4" apart.reroll dough scraps -- sprinkle a few chocolate jimmies over and gently pat into dough.
bake the cookies one sheet at a time, in middle of oven 6-9 minutes or just barely firm when pressed in centers.Reverse sheets from front to back halfway through baking time.
Transfer sheet to wire rack and let stand until cookies firm up slightly, 3-4 minutes.Using a spatula, transfer the cookies to wire racks, let stand until completely cooled --
Store in an airtight container for up to 2 weeks or freeze for up to 1 month.