Preheat the oven to 350℉ and line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the cake flour, baking powder, baking soda, and salt.
In the bowl of your stand mixer fitted with a paddle attachment (you can also use an electric hand mixer), cream together the room temperature butter, the brown sugar, and the granulated sugar. I like to set my mixer to medium speed and let it go for 2-3 minutes for this step.
Add the egg and vanilla extract to the butter/sugar mixture. Mix to combine.
Add the dry ingredients to the bowl and mix until it's just combined (don't over-mix!)
Roughly chop the white chocolate and add it to your cookie dough along with the rainbow sprinkles. Mix until combined.
Scoop the dough into 1" - 2" balls and place them a few inches apart on your baking sheet lined with parchment paper.
Bake at 350℉ for 8-10 minutes just until the edges are slightly golden.
Let the cookies cool on their tray before serving. Enjoy!
