I would use buttercream – Royal icing dries very hard and you may not get the look you’re after. If you are only using the buttercream to decorate the hair – I would use the Wilton recipe using Crisco only – adding butter will make it taste better, but will make the frosting softer. Here’s the recipe:
Ingredients: 1 cup Crisco shortening; 1 teaspoon clear vanilla extract (if you are tinting the buttercream – regular vanilla will do); 4 cups sifted confectioners’ sugar (approximately 1 lb.); 2 tablespoons milk (you may need a little more – depending on how thick you want the icing) – Makes: About 3 cups of icing.
Instructions: In large bowl, cream shortening. Add vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add milk and beat at medium speed until light and fluffy. Keep bowl covered with a damp cloth until ready to use. (Note, I usually have to add a little bit of the milk halfway through process).
For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Rewhip before using.
Optional: to enhance the flavor of the icing, you can add 1/2 teaspoon No-Color Butter Flavor.