Easy Grasshopper Sandwich Cookies

¼ cup butter or margarine, softened
2 tablespoons shortening
1 ¼ cups powdered sugar
1/8 teaspoon salt
2 tablespoons crème de menthe liqueur
1 (8 1/2–oz.) package chocolate wafer cookies

In small bowl, beat butter and shortening until combined. Add powdered sugar, salt and liqueur; beat until light and fluffy. Spread scant tablespoonful powdered sugar mixture on wafer cookie; pressing lightly. Repeat with remaining cookies and powdered sugar mixture. Refrigerate until set. 20 sandwich cookies.