How to Make It
Prepare the Cookies: Beat butter and sugar with a heavy-duty electric stand mixer on medium speed until fluffy, about 3 minutes. Set aside.
Stir together baking soda and hot water in a bowl until dissolved; stir in molasses.
Stir together flour, ginger, cinnamon, salt, and allspice in a large bowl. Add flour mixture to butter mixture alternately with molasses mixture, in 3 parts, beginning and ending with flour mixture. Beat on medium speed until incorporated after each addition. Shape mixture into a flat disk. Wrap in plastic wrap; chill 1 hour.
Preheat oven to 350°F. Roll dough to 1⁄8-inch thickness on a lightly floured surface. Cut dough with 3-inch and 4-inch cookie cutters. Gather scraps, and reroll as desired. Place Cookies 2 inches apart on parchment paper-lined baking sheets. Bake until edges are lightly browned, 15 to 18 minutes. Cool on baking sheets 2 minutes; transfer to wire racks. Cool completely, 30 minutes.
Prepare the Royal Icing: Beat powdered sugar, meringue powder, and 6 tablespoons of the warm water with an electric mixer on low speed until blended, 1 minute. Increase speed to medium; beat until smooth, 2 minutes. Beat in remaining warm water, 1⁄4 teaspoon at a time, until desired consistency is reached.
Spoon Royal Icing into a small ziplock plastic freezer bag with 1 corner snipped; pipe on Cookies.