Melt butter in a small saucepan over low heat, stirring occasionally. Alternatively, melt in microwave in 30-second intervals.
Pour melted butter into a large mixing bowl and set aside to cool for 10-15 minutes until it’s only slightly warm. Don’t let it firm back up again.
Add both sugars and whisk for 20 seconds until well combined. NOTE: The mixture will thicken gradually; avoid leaving it too long, as it will become harder to whisk the egg in the next step.
Add egg and vanilla, and whisk until well combined, about 15 seconds.
Sift over the flour, cornstarch, baking soda and pumpkin pie spice. Then add the salt.
Using a rubber spatula or wooden spoon, fold and mix until a soft cookie dough forms, it will be thick.
Chill the cookie dough in the refrigerator for 45 minutes, or if the bowl is too large, wrap the dough and refrigerate without the bowl. *For overnight chilling, see notes below.
While dough chills, preheat oven to 180ºC (356ºF). Line a large baking sheet with parchment paper and set aside.
After the 45 minutes, remove dough from fridge.
Scoop and weigh 45g (1.5oz) of dough to form each cookie; roll into tall balls and place on lined tray, without flattening the balls. NOTE: Without a food scale, use 3 level tablespoons per cookie.
Repeat with remaining dough, spacing cookie balls 10cm (4 inches) apart to allow for spreading, and leaving gaps near tray edges. Bake in batches, as they won't all fit on one tray.
Bake for 10-12 minutes until edges are lightly golden. Centers may look slightly under-baked; that's what keeps them soft and chewy.
Remove from oven and tap the tray on your bench top or stove top 3 times to help slightly deflate.
Allow cookies to cool on tray for 15 minutes.
Carefully transfer cookies to a wire rack to cool completely. They’ll still be soft and delicate so handle with care to avoid breakages.
Enjoy!