01 - Combine the sifted powdered sugar, meringue powder, and warm water in a mixing bowl. Beat on low speed with an electric mixer until the icing achieves stiff peaks, approximately 5 minutes. If the consistency is too thick, incorporate water a few drops at a time.
02 - Divide the prepared icing into several small bowls. Tint each portion with different food coloring gels to produce a vivid selection of colors.
03 - Using a small offset spatula or a piping bag fitted with a round tip, apply a white layer of royal icing to the top of each Oreo cookie. Let stand for 10 to 15 minutes for the icing to partially set.
04 - With colored icing and piping bags equipped with fine tips or toothpicks, adorn each cookie with classic sugar skull patterns—eyes, nose, mouth, as well as floral and geometric accents.
05 - Enhance each cookie with detail work using edible markers and adorn with assorted sprinkles, if desired.
06 - Allow all decorated cookies to rest at room temperature for a minimum of 2 hours, or until the icing is completely dry before serving or packaging.