Melt Butter: Over low heat in a saucepan, stirring occasionally. Alternatively, use a microwave in 30-second bursts. Transfer to a large mixing bowl and cool until slightly warm, about 5 minutes.
Prepare Wet Mixture: To the slightly cooled butter, add the both sugars and whisk for 20 seconds.
Add The Egg: Whisk in the egg and vanilla until well combined, about 15 seconds.
Add Dry Ingredients: Sift in the flour, cornstarch, and baking soda. Add the salt.
Combine: Mix with a spatula or wooden spoon. until a thick batter forms.
Chill Dough: Place the bowl in the refrigertaor and chill the batter for 45 minutes.
Preheat Oven: To 356ºF (180ºC) fan-forced. Line a baking sheet with parchment paper.
Form Cookies: Scoop 45g/1.5 oz (or 3 level tablespoons) of dough, roll into tall balls, and place on the tray, spaced 10cm apart. Do not flatten balls.
Bake: Bake for 10-12 minutes until lightly golden around the edges. Remove and tap the tray on your bench top or stove top 3 times to help slightly deflate.
Cool: Let the cookies sit on the sheet for 10 minutes, then move to a wire rack to cool completely. Enjoy!