Combine first 5 ingredients; stir well. Refrigerate overnight in an airtight container.
Combine eggs in a large mixing bowl, beating until frothy. Gradually add sugar, beating until thick and lemon colored.
Combine milk and oil. Sift flour, baking powder, and salt together; add to egg mixture alternately with milk mixture. Beat well after each addition. Stir in flavoring.
Pour batter into 3 waxed paper-lined and greased 8-inch round cake pans. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool layers in pans 10 minutes; remove from pans, and let cool completely.
Spread frosting between layers, and sprinkle each with 3/4 cup coconut-orange mixture. Spread remaining frosting on top and sides of cake. Top with remaining coconut-orange mixture, and garnish with orange slices. Refrigerate until serving time.