In a large bowl, whisk the melted butter with the sugar until creamy and no longer lumpy, then whisk in the pureed sweet potato.
Place a sieve over the bowl, and sift the flour, baking soda, baking powder, cinnamon and cardamom into the bowl.
With the help of a spatula mix to combine all the ingredients. Fold in the chocolate chips if using, and mix until combined.
Cover the bowl and refrigerate for 20-30 minutes.
Preheat the oven to 175°C/ 350°F and line two baking trays with parchment paper.
With the help of a cookie or ice cream scoop, scoop the dough and form cookie balls, then arrange them about 5 cm /3 inches apart on the baking sheets.
Bake for 11-12 minutes, or until the edges are just set and slightly crisp. Remove the trays from the oven and allow the cookies to cool and set for at least 10 minutes, then transfer them to a cooling rack and allow them to cool completely before serving.
