In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in eggs and extract. Combine flour and salt; gradually add to creamed mixture and mix well. Stir in lemon zest., Spread into a greased 13x9-in. baking pan. Bake at 350° for 18-22 minutes or until center is set and edges are golden brown. Cool completely., For frosting, in a large bowl, beat cream cheese and butter until fluffy. Beat in confectioners' sugar, lemon juice and zest. Frost top and cut into bars. Store in the refrigerator.