Melt the butter: Melt butter in a small saucepan over low heat, stirring occasionally. Alternatively, melt in the microwave in 30-second intervals.
Cool the butter: 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 the sugars: Add both sugars and whisk for 20 seconds until well combined.
Add the egg and vanilla: Add egg and vanilla, and whisk until well combined, about 15 seconds.
Add dry ingredients: Sift over the flour, cornstarch, baking soda, and pumpkin pie spice. Then add the salt.
Mix the dough: Using a rubber spatula or wooden spoon, fold and mix until a soft cookie dough forms, it will be thick.
Chill the dough: 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.
Preheat the oven: While dough chills, preheat oven to 356ºF (180ºC). Line a large baking sheet with parchment paper and set aside.
Portion the dough: After 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. NOTE: Without a food scale, use 3 level tablespoons per cookie.
Arrange cookies: 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 the cookies: Bake for 10–12 minutes until edges are lightly golden. Centers may look slightly under-baked, that’s what keeps them soft and chewy.
Tap the tray: Remove from oven and tap the tray on your bench top or stove top 3 times to help slightly deflate.
Cool the cookies: Allow cookies to cool on the tray for 10 minutes.
Transfer to a rack: Carefully transfer cookies to a wire rack to cool completely. They’ll still be soft and delicate, so handle with care to avoid breakages.