In a large bowl, cream the butter, shortening, sugars, eggs, vanilla, cream of tartar, lemon peel and salt with an electric mixer.
Add flour and cranberries.
Stir with a wooden spoon to combine.
Cover and refrigerate for 2 hours or until easy to handle.
Roll into 1-inch balls.
Place 2 inches apart on greased cookie sheets.
Flatten with a glass dipped in granulated sugar.
Bake at 350˚ for 12-14 minutes or until edges are lightly browned.
Rotate cookie sheets on oven racks after 6 minutes of baking time.
Remove to wire racks to cool.
Spread the bottom of one cookie with Lemon Buttercream Frosting.
Top with another cookie of similar size and shape.