Cream together butter, brown sugar, and granulated sugar until light and fluffy.
Add egg and vanilla extract and mix until combined.
In a separate bowl, whisk together flour, baking soda, baking powder, salt, and cornstarch.
Gradually add dry ingredients to wet ingredients and mix until combined.
Fold in rainbow sprinkles and chopped white chocolate.
Cover and refrigerate dough for at least 1 hour or up to 24 hours.
Preheat oven to 350°F (175°C).
Line baking sheets with parchment paper.
Scoop dough into 2-tablespoon portions and place on baking sheets.
Bake for 10-12 minutes until edges are lightly golden.
Cool on baking sheet for 5 minutes.
Transfer to wire rack and cool completely.