Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Melt the white chocolate in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Set aside to cool slightly.

In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the egg and vanilla extract.

Gradually mix in the melted white chocolate until well combined.

Sift together the flour, baking powder, baking soda, cornstarch, and salt. Gradually add to the wet ingredients, mixing just until incorporated.

Fold in the chopped macadamia nuts.

Drop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the edges are lightly golden. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.