Make the cookie: Line a baking sheet with parchment paper. Whisk the flour, ginger, cinnamon, allspice, nutmeg, salt and baking soda in a medium bowl.
Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Beat in the egg until just combined. Mix in the molasses and vanilla. Reduce the speed to medium low and beat in the flour mixture until just combined.
Turn out the dough onto the baking sheet. With lightly floured hands, press the dough into a 10-inch round, about 1/2 inch thick. Lightly cover with plastic wrap and refrigerate at least 30 minutes.
Preheat the oven to 350˚ F. Uncover the dough. Bake until the cookie is slightly puffed and golden around the edges, 18 to 20 minutes. Let cool 5 minutes on the pan, then slide the cookie (on the parchment) onto a rack to cool completely.
Meanwhile, make the icing: Whisk the confectioners’ sugar and meringue powder in a large bowl. Add 4 tablespoons water and whisk until the mixture is smooth and stiff peaks form. Add more water, 1 teaspoon at a time, until the icing is thin enough to pipe.
Transfer the icing to a piping bag with a small star tip. Pipe a snowflake design on the cookie. Sprinkle with granulated sugar and let the icing set, about 30 minutes. Cut the cookie into wedges.
You can turn this cookie into an edible holiday card: Just write your message in icing!