Preheat oven to 180°C/350°F. Line a large baking sheet with parchment paper.
In a medium mixing bowl, combine the plain flour, cornstarch, baking powder, baking soda and fine sea salt. Stir together using a whisk, to ensure all the ingredients are evenly distributed.
2 ½ cups all purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon fine sea salt
Melt the butter in a small saucepan. In the bowl of a stand mixer, combine the soft light brown sugar and granulated sugar with the melted butter. Mix on medium speed, until well combined. (Alternatively use a large mixing bowl and an electric hand mixer).
¾ cup unsalted butter, ½ cup soft light brown sugar, ½ cup granulated sugar
Add the egg, vanilla and lemon extracts and the lemon zest to the butter and sugar mixture and continue to mix on medium speed until well combined. Add the flour mixture and mix on low speed until no streaks of flour remain.
1 large egg, 1 teaspoon vanilla extract, 1 teaspoon lemon extract, 1 unwaxed lemon
Add the white chocolate chips to the cookie dough. Mix on low speed until evenly distributed throughout the dough.
7 ounces white chocolate chips or chopped white chocolate from a bar
Using a medium cookie scoop, divide the dough into approximately 2 tablespoon-sized balls (you should get 18-20 cookies from this mixture) and place on the prepared baking tray. Add a few extra chocolate chips to the tops if desired. You should get 6-8 on a tray at a time.
Bake in the preheated oven for 8-10 minutes (ours took 8 minutes in a fan-assisted oven) until lightly golden at the edges. They should still feel soft and underbaked in the centre – that’s what you want! They’ll continue to bake as they cool. Leave to cool on the tray for 5-10 minutes before transferring to a cooling rack to cool completely.