Puree the ginger in a mini food processor with 2 tablespoons water. Squeeze and strain juice through cheesecloth or a fine mesh strainer. Discard the ginger pulp. Reserve the juice.
Sift the flour, baking soda, salt and spices into a medium bowl, then whisk in the pepper; set aside.
Cream the butter and sugar with a handheld mixer until light and fluffy, about 1 1/2 minutes. Add the molasses and ginger juice and beat until evenly incorporated, about 2 minutes. (Stir together with a spatula if needed.) Gradually blend the dry ingredients on low speed into the butter mixture until you have a crumbly mixture. Bring the dough together by hand pressing and kneading lightly until moist, but not tacky. Divide into 2 equal portions, press into disks about 1/2-inch thick. Wrap in plastic and chill for at least 1 hour or overnight.
Position racks evenly in the oven and preheat to 350 degrees F.
Dust the dough and a work surface with flour. Roll the dough about 1/4 inch thick. Cut out cookies into desired shapes. Transfer to a parchment lined baking sheet. Bake until the cookies are a rich tawny brown, about 18 to 20 minutes. Cool on a rack.
For decorating: Whisk the water and egg white powder in a medium bowl until foamy and smooth. Gradually whisk in the confectioners' sugar to make a smooth icing. For multiple colors divide the icing into batches and add a food coloring as desired. Spread or pipe the icing onto the cookies. Finish with sprinkles, or candies. Let rest on a rack until the icing sets, about 30 minutes.
For a chewier cookie, roll them 1/3 to 1/2-inch thick and bake for 16 to 20 minutes.
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