preheat your oven to 175 degrees Celsius and line your baking tin with baking paper. Make sure you have enough on the sides that you can lift it out once it's cooled.
combine your flour, bicarb, salt and cornflour in a big bowl and set aside.
In a smaller bowl, add your caster sugar and brown sugar. Melt your butter and let it cool for a few minutes. Then add it to your sugar and mix it until it's smooth. Pay attention to any sugar lumps that may occur.
Then, add your egg and vanilla paste and whisk until all fully combined.
Pour your sugar mixture into the dry ingredients and stir until it's completely mixed together.
Add in your chocolate chips and mix until the chocolate chips are well distributed into the mix.
Push the mixture into your lined cake tin (it will be a little wetter that regular chocolate chip cookie mix). Use your hands to firmly distribute the dough across the tin and make sure to push it into the corners.
Cook for 30 minutes or until golden brown on top. Once you take it out of the oven, sprinkle some sea salt on the top if you like sweet and salty.
Let it cool for an hour and then remove from the tin and slice. Store in an airtight container in the pantry.