Sift together the flour, cocoa, baking powder and salt - set aside.
In a medium bowl combine the butter and sugar and beat until creamy and smooth. Beat in the egg. Gradually stir in the sifted ingredients for form a soft dough. Divide the dough in two pieces, flatten and wrap in wax paper or parchment. Refrigerate for 2 hours.
Preheat the oven to 375 degrees. Flatten out the wax or parchment paper you used to wrap the dough and put another piece over top. Full out to about 1/8 inch thickness.
Cut into desired shapes with cookie cutters. Dough is sticky so the paper helps.
Line a cookie sheet or two (depending on the size of your cookies) with parchment paper. Place cookies 1 1/2 inches apart on cookie sheets.
Bake for 8 to 10 minutes in the preheated oven. Cool completely before attempting to glaze.
Meanwhile, combine the glaze ingredients in a medium bowl and whisk until completely incorporated. Put the glaze into a plastic bag, snip off a bit of one corner, and decorate as you like.