With a stand or hand mixer, cream together the butter and sugar for about 2 minutes until light and fluffy. Incorporate the egg and vanilla extract, mixing until thoroughly blended. In a separate bowl, whisk together the flour, baking powder, baking soda, corn starch, and salt. Gradually add the dry mixture to the creamed butter, beating on low speed until just incorporated. Gently fold in the chopped macadamia nuts, ensuring not to overmix. Delicately fold in the white chocolate. Cover the dough and refrigerate for one hour. Scoop generous tablespoon portions of dough and arrange them on a parchment-lined baking sheet, leaving about 2 inches between each. Press additional white chocolate and macadamia nuts onto the tops and sprinkle with flaked salt if desired. Bake at 350°F for 10 to 12 minutes, until the edges are lightly golden. Allow the cookies to cool on the pan for 10 minutes before transferring to a wire rack to cool completely.