In a large bowl, cream 3/4 cup of unsalted butter and 3/4 cup of granulated sugar at a medium high speed for about 2 minutes until the mixture becomes lighter in color.
Mix in 1 large egg and 1 tsp vanilla, scraping down the sides of the bowl as needed with a rubber spatula.
Add in 1/4 tsp salt, 1 tsp baking powder, 1/2 baking soda, and 1 tsp of cream of tartar or cornstarch. Mix together on a low speed until the ingredients are just incorporated.
With the mixer running on low speed, slowly add 1 1/2 cups of all-purpose flour into the batter in two additions. The cookie dough will be quite thick, but that's how it's supposed to be.
The last step is to add in 1/2 cup of rainbow sprinkles and 1/2 cup of crushed oreos. If you want to add in a bit more chocolate, you can also fold in 1/2 cup of mini chocolate chips or chopped up chocolate chips. Mix them into the dough on a low speed until they're evenly distributed..
Scoop and roll cookie dough balls that are about 3/4 inch in diameter. Place the balls of dough on cookie sheets lined with parchment paper or a silicone baking mat and chill for an hour in the fridge. This helps make the cookies chewy and prevents them from spreading too much while baking!
After 40 minutes of chilling your cookie dough, preheat your oven to 350 F.
Rearrange the cookie dough balls on the baking mats so that they're 2 inches apart from each other.
Bake the chilled cookie dough for about 9-11 minutes. The cookies will appear undone and soft/puffy, but will continue to bake and firm up while they cool on the baking sheet.
If desired, press a few pieces of crushed oreos and sprinkles on top of each cookie once they've been out of the oven a few minutes.
Allow the cookies to cool on the cookie sheets for 10 minutes, then move them to wire rack to cool completely.