Preheat oven to 350 degrees. Combine butter, sugar, egg yolks, zest and orange extract in a large mixing bowl. Cream the ingredients together thoroughly.
Sift flour, salt and baking powder together in a separate mixing bowl. Add dry ingredients to creamed ingredients a third at a time alternately with the orange juice. Fold in the beaten egg whites. Spoon batter into cupcake liners until half full.
Bake for 15 minutes or until a toothpick inserted in the center comes out clean. When cupcakes are cool, frost with vanilla buttercream frosting (below) and garnish with orange slice candies.
Frosting: In a large bowl, cream butter and vanilla extract. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides of bowl often. Add milk and beat at medium speed until light and fluffy.
Cook's Note: For best results, keep icing in refrigerator when not in use. This icing can be stored for up to 2 weeks in the refrigerator. Bring to room temperature and re-whip before using. Makes 3 cups of icing.