Cream butter, shortening, and sugar until light and fluffy; beat in lemon zest and egg. Stir in lemon juice and water.
Sift together the flour, soda, cream of tartar, and salt. Stir into the first mixture. Stir in chopped pecans, if using. Flour hands and shape dough into small balls. Place balls about 1 inch apart on ungreased baking sheet. Bake for about 10 to 15 minutes in a preheated 350° oven. Cookies should be browned on the bottom, but not on top. Immediately remove hot cookies from the baking sheet and roll in confectioners' sugar. Makes about 3 to 4 dozen cookies, depending on size.