Center a rack in the oven and preheat to 350 (F). Prepare a sheet pan by lining with parchment paper; set aside.
In a medium bowl, combine 1 2/3 cups all-purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, and 1 teaspoon kosher salt. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 1/2 cup unsalted butter, 3/4 cup dark brown sugar, and 1/4 cup granulated sugar. Beat on medium-high speed for 5 minutes, until the mixture is light, fluffy, and has doubled in volume.
Once the mixture is light and fluffy, reduce the mixer to its slowest speed. Add 1 large egg and 1 teaspoon pure vanilla extract, mixing until both have just incorporated into the butter and sugar. With the mixer still on its slowest setting, add the dry ingredients (from the 2nd step) in three big portions, only beating until the flour has just incorporated in to the mixture. Add 6 ounces rainbow chocolate nonpareil discs and 4 ounces roughly chopped dark chocolate, beating on the mixer's lowest speed just until the chocolate has incorporated throughout the dough.
Use a 1-tablespoon sized cookie dough scoop to portion out 3 ounce dough balls. Use your hands to roll them into a rough ball, straightening and flattening any nonpareil discs accordingly (this will make prettier cookies, I promise). Press the top of each ball to flatten it and create a sort of squat disc. Arrange on the parchment lined sheet pan, leaving at least 3 inches between each dough ball.
Transfer to the preheated oven and bake for 18-20 minutes, or until the edges are set and the tops are brown, but the center is still a little bit doughy. Be careful not to overbake the cookies, or they'll be rough and crunchy and I'll cry! Unless you like a crunchy cookie. In which case, I won't.
Once the cookies are done, transfer to a wire rack and allow to cool on the sheet pan for 20-25 minutes, before using a spatula to lift them out the pan and transferring them to the wire rack to cool until warm enough to eat. Enjoy!