In a medium bowl, mashed the softened butter into the brown sugar, granulated sugar, and flour to make the streusel topping. Set aside.
6 tablespoons salted butter, ¼ cup brown sugar, ¼ cup granulated sugar, ⅔ cup all-purpose flour
Preheat oven to 350°F. Line two baking sheets with parchment paper.
Beat butter, granulated sugar, and brown sugar until creamy and light. Add the egg, egg yolk, and vanilla extract, mixing until combined. Stop to scrape the bottom and sides of the bowl.
½ cup salted butter, ½ cup granulated sugar, ¼ cup brown sugar, 2 teaspoons vanilla extract, 1 large egg
Add flour, cornstarch, baking powder, baking soda, and salt, mixing just until no streaks of flour remain.
1 ¾ cups all-purpose flour, 1 tablespoon cornstarch, 1 teaspoon baking powder, ¼ teaspoon baking soda, ¼ teaspoon salt
Fold in blueberries and white chocolate chips by hand.
1 cup fresh blueberries, 1 cup white chocolate chips
Scoop large mounds of cookie dough to make 10 cookies, spacing them a few inches apart on the baking sheets so they have room to spread. Mound some of the streusel on top of each cookie.
Bake for 11-13 minutes until set and lightly golden brown around the edges. Do not overbake. Let cool on baking sheets.