In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the room-temperature butter and caster sugar together on medium speed for about 3-4 minutes, until the mixture is light, pale, and fluffy. Scrape down the sides of the bowl as needed.

Add the whole egg, egg yolk, and vanilla extract. Beat again on medium speed for another minute until everything is well combined and the mixture is smooth.

In a separate medium bowl, whisk together the plain flour, cornflour, baking powder, and salt. This ensures the leavening agents are evenly distributed.

With the mixer on its lowest speed, gradually add the dry ingredients to the wet ingredients in three additions. Mix only until the last streaks of flour have just disappeared. I find it’s best to stop the mixer early and finish folding with a spatula to avoid overworking the gluten, which can make the cookies tough.

Cover the bowl with cling film and chill the dough in the refrigerator for at least 1 hour, or up to 3 days. This step is essential for preventing the cookies from spreading too much in the oven.

When you're ready to bake, preheat your oven to 180°C (160°C Fan) and line two large baking trays with parchment paper.

Pour some extra caster sugar into a small, shallow bowl. Scoop the chilled dough and roll it into balls about 1.5 inches in diameter. Roll each ball in the caster sugar to coat it completely.

Place the sugar-coated balls onto the prepared baking trays, leaving about 5-6cm of space between each one to allow for some spreading. Gently press down on the top of each ball with the palm of your hand or the bottom of a glass to flatten it slightly.

Bake for 10-12 minutes, or until the edges are a very light golden brown and the centres look set but still soft. They might appear slightly underbaked in the middle, but they will continue to cook on the hot tray.

Let the baked cookies cool on the baking tray for 5 minutes before carefully transferring them to a wire rack to cool completely.