In a large mixing bowl, combine cold butter chunks with both brown and caster sugars. Blend until just mixed, ensuring the mixture remains slightly lumpy.

Gently fold in the pumpkin purée, and gradually add the all-purpose flour and homemade pumpkin spice mix. Avoid overmixing; the dough should be clumpy.

If the dough feels sticky, cover it with plastic wrap and chill in the refrigerator for about 15 minutes.

Preheat your oven to 190°C (375°F). Roll the dough into golf ball-sized balls and place them on a lined baking sheet.

Freeze the dough balls for at least 2 hours—overnight is best.

Arrange the frozen dough balls on a baking sheet, leaving space between each. Bake for 16 minutes or until golden brown.

Allow the cookies to cool on the baking tray for 20 minutes before serving.