Preheat the oven to 325ºF. Line a baking sheet with a silpat liner.

Cut the butter into 1 tablespoon sized pieces and place in a microwave safe bowl. Microwave for 45 seconds. You don’t want the butter to be completely melted, but you want it almost melted. (Refer to the video if you need a visual of how the butter should look.)

Place the butter and both sugars in the bowl of a stand mixer. (You can also use a large bowl and a hand mixer, if desired.). Beat until very well combined. Beat in the vanilla extract. Add the egg and beat just until combined.

Add the flour, cornstarch, baking powder and salt. Slowly beat until all the ingredients are combined. *If the dough is still sticky, start adding more flour, about 2 tablespoons at a time, until the dough is no longer sticky and will easily form a ball.

Add in the chocolate chunks and mix to combine.

Divide the dough into 5 balls - each should be about 1/2 cup. Make the balls a little taller than they are around, and place on the baking sheet.

Bake in the preheated oven until the cookies are browned, about 15 minutes.

Let the cookies cool on the baking sheet for about 15 minutes then serve, if you are wanting the cookies warm and gooey.