Preheat oven to 350 F / 180 degree celcius.
Cream together butter and sugars until smooth.
Add egg and vanilla and blend in.
Mix the rest of the dry ingredients and add in chocolate chunks.
Use a tablespoon to drop cookie dough on to a greased baking sheet.
Bake for approx. 8 to 10 minutes or until golden brown around the edges.
Once done, let it cool on a wire rack and enjoy.

*What makes the cookies chewy is the CORNSTARCH or Tapioca Flour.
* To make CRISPY COOKIES - Omit the cornstarch and replace the baking soda with baking powder.