Preheat oven to 350 degrees and line a baking sheet with parchment or silpat.
Mix the softened butter, brown sugar and sugar together until well blended. Add in the egg yolk and vanilla, mixing well.
Add in the flour, baking powder, baking soda and salt. Mix until most of the flour is blended in, and then add in chocolate chips and walnuts (if using).
Divide mixture into four equal portions (roughly 1/4 cup each). Tip with flaked sea salt and any additional chocolate chips. Bake at 350 degrees for 12-14 minutes or until the edges turn golden brown. Cool on wore rack.