Whisk together the flour, cornstarch, baking powder and salt
In a separate bowl, cream the butter, brown sugar, granulated sugar and vanilla together. Cream on medium speed, scraping down the sides every few minutes, until the butter has visibly lightened in color and looks fluffy. Add 1 tbsp of milk at a time and beat into the butter mixture until smooth. This should take about 5-6 minutes total.
Fold the dry ingredients into the wet until just combined, then add the chocolate and mix.
Chill the dough in the fridge for 1-4 hours.
When you are ready to bake, preheat the oven to 350F.
Split the dough into 12 even balls, weighing about 45g each.
Place on parchment lined baking sheets and bake for 12-14 minutes.
Take it out of the oven when they still seem slightly underdone, then let them cool completely **this is the most important step!**
Serve whenever!