To make the cookie dough: Sift together flour, baking powder, and salt. Set aside. Place butter and sugar in large bowl of electric stand mixer and beat until light in colour. Add egg and milk and beat to combine. Put mixer on low speed, gradually add flour, and beat until mixture pulls away from the side of the bowl.
Dust a sheet of parchment paper with flour. Place the dough on top. Dust with more flour and top with another sheet of parchment. Roll it out to 1/8-inch thickness and chill until firm, about 20 minutes.
Cut the pieces: Measure and cut six panels for each box: a 4' x 4' top and bottom, two 4' by 2' sides and two 3 3/4' by 2' sides. Use a sharp knife or pizza cutter to cut them out.
Bake all pieces for 10 minutes and leave to cool. To create the perfect right angles after cooling, use a fine kitchen rasp to file the edges, making them nice and straight.
Thicken some royal icing: Thicken about 2 cups of the royal icing with either cornstarch and a couple of drops of vinegar, or extra icing sugar, to get the consistency of caulk. Fit a pastry bag with a medium round tip and fill it with the thickened icing.
Make the boxes: Using royal icing, attach the four walls to the bottom and to each other. The longer walls will face each other. Leave the tops for the end. Let the boxes dry 24 hours before adding finishing touches. Store remaining royal icing with plastic wrap touching the surface.
When dry, use a paring knife to trim off excess overflowing icing so the outside of the box has a clean finish.
Decorate: Fit small round tips into piping bags. Divide the remaining royal icing into batches and color as desired. Fill bags with icing and pipe neat lines along the seams of the box first. Then go back and pipe little dots along the piped lines to create more interest. At this point, you can decorate the box and lid however you want. Do some pencil sketches on paper first to get an idea of what you might like. Then go for it.
Let the icing dry thoroughly for at least 24 hours. Fill with treats of your choosing, tie with a ribbon and present.
From Food Network Kitchen