Preheat oven to 350 degrees. Line muffin tin with cupcake liners. In mixing bowl, combine cake mix, orange juice, oil, eggs, and coconut. Beat with mixer for 2 minutes on medium speed. Pour into cupcake liners, filling each about 2/3 full. Bake for 18 to 22 minutes, or until toothpick inserted in center of cupcake comes out clean. Remove cupcakes from muffin tin and place on wire rack; cool completely. Frost cupcakes with Orange Buttercream Frosting; sprinkle with toasted coconut and garnish with orange slice, if desired.
In mixing bowl, combine all ingredients. Beat on low speed with mixer. Increase speed gradually and beat until smooth and creamy, scraping down sides of bowl often.