Preheat the oven to 325°F(165°C). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Place the flour, cornstarch, baking soda and salt if using in a medium bowl and mix well. Set aside.
Melt the butter halfway in a small pan over medium heat. Remove from the heat and stir until it is all melted. Set aside to cool.
Place the sugar in a big bowl and add the butter, mix well. Then add in the egg and stir until combined. Don't overmix. Mix in the vanilla.
Add the flour mixture in 2-3 batches and stir with a spatula until all combined well. Don't overmix.
Stir in the chocolate chips evenly.
Roll the dough into 40 gram (2 tablespoons) balls, and place them on the baking sheet. Add the extra chocolate chips to decorate.
Bake for almost 9 minutes or until the cookies are slightly set. Check the oven after 8 minutes. The top of the cookies will be soft but not wet. These cookies will be too soft to handle when you remove from the oven but they’ll be firm as they cool on the baking sheet for 10 minutes.