Into large mixing bowl measure flour, butter, walnuts, vanilla, baking powder, powdered sugar, salt, & cardamom.
Knead with hands or mixer until well blended. If you don't have a stand mixer I'd recommend kneading this with your hands. The warmth of your hands seems to speed the incorporation of the butter into the dough. The dough is properly mixed when it pulls away from the bowl and your hands or the beater(s).
Chill the dough covered with cling wrap or in a sealable plastic bag in the fridge for at least 4 hours or overnight. (The dough will also keep well in a sealed plastic bag or container in the freezer for up to a month.)
When ready to bake preheat oven to 350F or 180C. Roll the chilled dough into 1 inch balls placing them about 2 inches apart on a parchment or silicone mat lined baking tray.
Bake for 20 minutes or until browned on the bottom. Immediately remove cookies from tray with spatula & place on wire racks to cool. If you like, roll cookies in powdered sugar while still warm. Store tightly covered in airtight container for up to 3 weeks. This recipe makes approximately 24 cookies.