In a mixing bowl, beat together the butter, coconut sugar, and cane sugar until creamy. Add the vanilla and egg and egg yolk to to the bowl and continue to mix well.
In a separate bowl, whisk together the King Arthur Measure for Measure Gluten Free Flour, corn starch, baking soda, baking powder, and sea salt. Add the flour mixture slowly to the mixing bowl and mix just until the flour is incorporated.
Fold in the chocolate chips and chopped walnuts. Measure 6 oz cookie dough scoops and place on a baking sheet. Place the baking sheet in the fridge to cool for at least an hour.
When ready to bake, preheat the oven to 400 F and remove the baking sheet from the fridge. Bake the cookies in the preheated oven for 12 - 14 minutes until the outside has turned a golden brown.
Let the cookies sit for at least 10 minutes before serving.