Preheat oven to 350°F. Line two cookie sheets with parchment paper and set aside.
In a standing mixer fitted with a paddle attachment, cream the butter and sugars until light and fluffy, about 3 minutes. Beat in the egg and vanilla extract for an additional minute.
In a separate bowl, whisk together the flour, cornstarch, salt, baking soda, and baking powder. With the wet ingredients mixing on low speed, slowly incorporate in the dry ingredients until almost combined. Remove from mixer and fold in oats and white chocolate chips with a spatula. Once incorporated fully, carefully fold in blueberries, making sure not to break them!
Using a 3 oz cookie scoop, scoop the dough onto the cookie sheet, leaving 2 inches in between each cookie. Bake cookies for 19 minutes, or until just beginning to get light golden brown. Allow to cool on cooling rack before serving!