1. Allow the butter to soften and cream it until light. Beat in the sugar in a stream and continue to beat until the mixture whitens. Add the egg whites, one at a time, and continue beating until very creamy. Sift the cocoa powder and flour together several times, combine with the ground walnuts and stir into the batter.

2. Pipe or spoon 1-inch spheres of the batter on paper-lined pans. Bake at 350 degrees about 10 minutes. Remove the paper from the pans and cool the cookies.

3. For the filling, heat the cream until hot to the touch. Cut the chocolate finely and stir into the cream off the heat. Stir until the ganache is smooth. Strain the ganache into a mixing bowl and allow to cool at room temperature until thickened. Beat the ganache by hand for a minute, beating in the liqueur in a stream.

4. Turn half the cookies over so that the flat side is up. Pipe a dab of the ganache on each one. Top with the other cookies, pressing them well together.

5. Pipe a tiny dab of filling on the top of each cookie. Press a small pecan half against the filling. Or streak the cookies with melted chocolate and sprinkle with a pinch of ground walnuts.