The rich, crunchy cookies call out for a glass of milk.
Sift flour, cocoa powder, baking powder, and salt into a large bowl. Set aside. Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until creamy. Mix in egg and vanilla. Reduce speed to low. Add flour mixture; mix until just combined. Halve dough, and wrap in plastic. Refrigerate about 1 hour.
Preheat oven to 325 degrees. Roll dough on a lightly floured surface to 1/8 inch thick. Cut into 1 inch squares. Space 1 inch apart on baking sheets lined with parchment paper. Refrigerate 30 minutes.Wrap scraps in plastic, and refrigerate 30 minutes; reroll and cut. Bake until edges just begin to brown, about 12 minutes. Cool on sheets on wire racks.
Put 9 ounces chocolate in medium bowl. Bring cream to a boil. Pour over chocolate; let stand 5 minutes. Stir until smooth. Chill until spreadable, about 1 hour, stirring occasionally. Spread onto bottom of half the cookies. Sandwich with remaining cookies.
Melt remaining chocolate in a small heatproof bowl set over a pan of simmering water. Fill a parchment paper cone, or a small pastry bag fitted with a round tip (such as Ateco #2), with melted chocolate; decorate cookies.
