Preheat the oven to 350℉. Line baking sheets with parchment paper and set aside.
In a large bowl, cream together the butter and sugars until light and fluffy. Mix in the egg and vanilla.
In a separate bowl, mix together the flour, cornstarch, baking soda and salt.
Slowly mix the dry ingredients into the wet ingredients until fully incorporated. Fold in the white chocolate chips and cashews.
Using a medium cookie scoop, scoop the cookie dough onto the prepared baking sheets and bake in the preheated oven for 8-10 minutes, or until lightly browned. Allow to cool on baking sheets for around 5 minutes before transferring to wire racks to cool completely.