1. Preheat oven to 325° F.
2. Beat butter, sugar, orange peel, vanilla, and baking powder in a large bowl with mixer on medium speed until fluffy. Beat in egg. Reduce mixer speed to low and gradually add flour, beating just until blended.
3. Divide dough in half. Shape each half into a 10 inch long log. Press down on each log to flatten 1 side completely.
4. Wrap and freeze at least 30 minutes or until firm enough to slice. (If frozen too hard, let thaw at room temperature 10-20 minutes.)
5. Cut 1 log at a time into 40 ¹/₄-inch thick slices. Place 1 inch apart on ungreased cookie sheet.

Icing:

1. Whisk ingredients in a medium bowl until thick and smooth. Spoon into a gallon size ziplock bag. Pour colored sugar into a bowl. Snip off top corner of bag and pipe icing on cooking to resemble orange slices. Gently press cookies, icing side down into orange sugar. Shake off extra.
2. Bake 11 to 13 minutes until very lightly browned. Remove to a wire rack to cool completely.