In a stand mixer, or in a medium-sized bowl with a hand mixer, cream the butter and sugar until pale and smooth, about 10 minutes. On low speed sift in the flour and pinch of cardamom, beating in slowly until just combined.
Cover the bowl with plastic and refrigerate for 30 minutes. Preheat the oven to 170C / 325F and line a baking sheet with parchment paper while the dough rests. Form the dough into balls, a little larger than a marble.
Place on the cookie sheet, flatten gently with your fingertips, and press a nut in the middle if using. Continue until all the dough is used. Bake 10 minutes until the cookies are just set, but not browned — the goal is to maintain their ivory color. Let cool completely on the tray or on a wire rack before serving.