Kids will love these sweet notes made of gingerbread. Use royal icing to create an envelope design or address the letter to Santa directly.
Cookies: Whisk together flour, baking soda, baking powder, spices, and salt.
Combine butter and brown sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed until light and fluffy, about 5 minutes. Add eggs, one at a time, beating to combine after each addition and scraping down bowl as needed. Beat in molasses. Add flour mixture and beat on low until just combined. Divide dough in thirds; form each into a flat rectangle and wrap in plastic. Refrigerate until cold, at least 1 hour and up to 2 days.
Preheat oven to 350 degrees. Roll out dough 1/8 inch thick between lightly floured sheets of parchment, flipping and lifting off parchment occasionally to avoid sticking. Transfer to a baking sheet; chill 30 minutes. Cut into 3-by-2 1/4-inch rectangles. Chill until firm, 15 minutes.
Bake until crisp but not dark, about 14 minutes. Halfway through baking, lift up sheet and bang on oven rack to flatten cookies. Let cool on sheets on wire racks.
Royal Icing: Beat confectioners' sugar, meringue powder, and a scant 1/4 cup water in the bowl of an electric mixer fitted with the paddle attachment on low speed until smooth, about 7 minutes. If icing is too thick, add more water, 1 teaspoon at a time; if too thin, beat icing for 2 to 3 minutes more, or add more sugar, 1 tablespoon at a time. Use immediately or store in an airtight container at room temperature overnight; stir before using. (Makes about 2 1/2 cups.)
Transfer icing to a piping bag fitted with a very small round tip (such as Ateco #1 or #2). Pipe designs on cookies. Let cookies stand at room temperature until set, at least 2 hours (preferably overnight).