Melt chocolate and butter in a metal bowl set over a saucepan of simmering water, stirring until smooth.
Whisk together eggs, walnuts, Sambuca, and granulated sugar in another bowl.
Stir in flour mixture and chocolate (dough will be thin).
Cover and chill until firm, about 2 hours.
Preheat oven to 350 degrees F.
Sift confectioners sugar onto a plate.
Roll heaping tablespoons of dough into balls and roll in confectioners sugar to generously coat.
Arrange balls 2 inches apart on 2 lightly greased baking sheets and bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until puffed and cracked but centers are still a bit soft, 10 to 12 minutes total.