In a large bowl, whisk together flour, cocoa powder, baking powder and salt. In a second large bowl, beat butter and sugar until smooth, about 2 minutes. Beat in egg and orange and vanilla extracts. On low speed, beat in flour mixture until just combined.
Form dough into 3 logs, each 8 inches long. Wrap in plastic and refrigerate 4 hours or as long as overnight. Trim ends and cut logs into 1/4-inch-thick slices (24 each). Place rounds on ungreased baking sheets. Bake at 350°F for 10 to 12 minutes. Remove to wire rack to cool completely.
Meanwhile, in a large bowl, beat confectioners' sugar, softened butter, orange juice and orange extract until good spreading consistency. Spread flat side of half the cookies with filling. Top each with a plain cookie. Refrigerate 1 hour. Dust with confectioners' sugar.