Cream the softened butter and sugar together until well combined.
Add the egg and vanilla extract. Mix until smooth.
In a separate bowl, whisk together the flour, baking powder, baking soda, cornstarch, and salt.
Add the dry ingredients to the wet ingredients. Gently fold together.
When the dough is almost completely combined, fold in the macadamia nuts and white chocolate.
For thicker cookies, cover and chill the dough for 30–60 minutes.
Preheat the oven to 350°F and line a baking sheet with parchment paper.
Scoop the dough onto the prepared baking sheet, leaving space between each cookie.
Bake for 10–12 minutes, depending on the size. Remove them when the edges are set but the centers still look slightly soft.
Let the cookies cool on the baking sheet for about 10 minutes before transferring them to a cooling rack.