Chop the butter into pieces and add to either a heatproof bowl or small saucepan. Add in the instant espresso powder/coffee powder.
Melt the butter either in a saucepan over low heat or in the microwave using short 30-second intervals on medium power, stirring between each interval. The espresso powder will start to soften, but not necessarily dissolve.
Once the butter is melted, let the mixture sit for 10 minutes.
Add the melted butter with espresso powder to a large bowl. Add in the sugars and beat until combined. You don't want to see pools of melted butter.
Beat in the egg and vanilla extract.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt.
Add the flour mixture into the butter mixture. Start with the mixer on a low speed and gradually increase to a medium speed until combined.
Stir in the chocolate chunks.
Cover the bowl with plastic and refrigerate for at least 2 hours or overnight.
When ready to bake, preheat the oven to 350F (180C or 170C on a fan forced oven). Line cookie sheets with parchment paper.
Form the dough into balls with about 3 tablespoons of dough each. I used 2 scoops of a medium cookie scoop (mine holds 1.5 tablespoons of dough) or you could use a large cookie scoop.
Place dough balls 2-3 inches apart on the lined cookie sheets.
Bake 1 sheet at a time in the middle of the oven for 10-12 minutes, or until the tops look set.
Cool cookies on the cookie sheet for at least 10 minutes before transferring to a wire rack to continue cooling.