Step 1

For the cookies: Whisk together the flour, cocoa powder and salt in a medium bowl. In a large bowl, beat together the butter and powdered sugar with a mixer on medium speed until light and fluffy. Add the egg and vanilla and beat to combine. Reduce the mixer speed to low and slowly beat in the flour mixture until fully incorporated. Form the dough into a rectangle and wrap in plastic wrap. Refrigerate until firm, at least 1 hour.

Step 2

Cut the dough in half. On a lightly floured piece of parchment paper, roll out 1 piece of dough until ¼ inch thick. Slide the dough (on the parchment) onto a baking sheet and freeze until firm, about 10 minutes. Repeat with the other piece of dough.

Step 3

Preheat the oven to 350˚. Slide the dough back onto a work surface. Cut out cookies with a snowflake cookie cutter, rerolling the scraps as needed. Reline the baking sheets with parchment paper. Arrange the cutouts about 2 inches apart on the baking sheets. Freeze until firm, about 15 minutes.

Step 4

Bake the cookies, rotating the pans halfway through, until crisp, 12 to 14 minutes. Let cool completely on the baking sheets.

Step 5

For the icing: Beat the powdered sugar and egg whites in a large bowl with a mixer on low speed until combined. Gradually beat in the water until the icing is pipable.

Step 6

Scrape the icing into a piping bag fitted with a small round tip. Pipe designs onto the cookies and immediately sprinkle with the sanding sugar. Let set about 1 hour.