In a large mixing bowl beat butter and shortening with an electric mixer on medium to high speed for 30 seconds. Add granulated sugar, sour cream, egg, vanilla, lemon and lime peels (if using), baking powder, baking soda, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in as much of the flour as you can. Stir in remaining flour. Divide dough in half. Cover and chill about 2 hours or until easy to handle.
On a lightly floured surface, roll half of the dough at a time to 1/8- to 1/4-inch thickness. Using cookie cutters or a sharp knife dipped in flour, cut dough into circles, triangles, squares, and/or diamonds. Using straws or aspic or hors doeuvre cutters, cut random holes in the cutout shapes so they resemble snowflakes. Use a straw to cut half circles from the edges to flute them. Using a wide spatula, place cookies about 1 inch apart on an ungreased cookie sheet. Sprinkle with edible cake glitter or colored sugar.
Bake in a 375 degree F oven for 7 to 8 minutes or until edges are firm and bottoms are very lightly browned. Transfer cookies to a wire rack; cool. When cookies are cool, sprinkle with powdered sugar. Makes about 40 (2-1/2-inch) cookies.