In a large bowl, beat the remaining 10 tablespoons butter and 2/3 cup granulated sugar on medium until combined. Beat in egg whites, vanilla, 4 teaspoons of the lemon zest and the remaining 2 tablespoons lemon juice (mixture may appear slightly curdled). In a small bowl, stir together the next four ingredients (through salt). Add to butter mixture and beat just until combined. If necessary, chill dough 1 hour or until easy to handle.