Add the sugar, egg, and vanilla extract, and beat until well combined.
In a separate mixing bowl, whisk together the flour, cornstarch, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Stir in the white chocolate chunks and blueberries.
Use a cookie scoop or spoon to form balls of dough, and place them onto the prepared baking sheet.
Bake for 10-12 minutes, or until the edges are lightly golden.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
In a small saucepan, heat the blueberry jam over medium-low heat until melted.
Drizzle the melted jam over the cooled cookies.
Dust the cookies with powdered sugar before serving, if desired.
Enjoy!