Preheat oven to 350° F (180 ° C). Line a 9x9 pan with parchment paper and set aside.
In the bowl of a stand mixer, cream together the butter and sugars until smooth, light in color and fluffy. This will take a few mintues.
Scrape the sides of the bowl and mix in the egg add vanilla.
Once completely combined, stop the mixer and add in the flour, baking soda and salt. Mix until combined.
Once the dry ingredients are mixed in, add in the chocolate chips/chopped chocolate and mix until thoroughly distributed. Gently press the mixture into your pan. Top with more chocolate, if desired.
Bake at 350° F (180° C) for 25-30 minutes or until the tops of the cookie bars are golden brown.
Remove from the oven and allow the cookie bars to cool completely before cutting and serving.
Store in an airtight container for up to 3 days or in the freezer for up to 3 months.