Melt chocolate in top of double boiler over hot, not boiling, water.
Remove from heat; cool.
Cream butter and 1 cup sugar in large bowl until blended.
Add egg yolks, vanilla and melted chocolate; beat until light.
Blend in flour to make stiff dough.
Divide dough into 4 parts.
Shape each part into a roll, about 1 1/2 inches in diameter.
Wrap in plastic wrap; refrigerate until firm, at least 30 minutes or up to 2 weeks.
Preheat oven to 350°F.
Line cookie sheets with parchment paper or leave ungreased.
Cut rolls into 1/8 inch-thick slices; place 2 inches apart on ungreased cookie sheets.
Bake 8 to 10 minutes or just until set, but not browned.
Remove to wire racks to cool.
Dust with powdered sugar.