Preheat oven to 325 degrees. Cream together the butter, shortening and sugars and vanilla until the mixture is fluffy. Sift together the flour, cardamom, baking powder and salt. Blend into the butter/sugar mixture. Fold in the chopped nuts. Roll teaspoons of dough into balls, place balls onto greased baking sheet, and flatten each ball with the bottom of a juice glass or cookie press dipped in granulated sugar. Arrange slivered almonds in a decorative star pattern on top of each cookie. Bake the cookies 18 minutes or so. Remove from cookie sheet and cool.