In a large bowl, cream butter, shortening and sugar until light and fluffy. Beat in egg, milk, lemon zest, lemon juice, and extract. In a separate, medium bowl, combine the flour, salt and baking soda; gradually add to creamed butter mixture.
Shape into 1-inch balls or drop by rounded teaspoonfuls, 2 inches apart onto ungreased baking sheets lined with silicone mats or parchment paper. Flatten with a glass dipped in sugar.
Bake at 400° F for 9-11 minutes, or until edges are lightly browned. Remove to wire racks to cool.