In a medium bowl, sift together flour, baking soda, baking powder, and kosher salt.

In a large mixing bowl or the bowl of a stand mixer, whip butter until light and fluffy.

Add sugars and mix on medium-high speed until well combined.

Mix in the peanut butter, egg, and vanilla extract.

Add the dry ingredients and mix on medium speed until just combined.

Stir in the mini M&Ms, semi sweet chocolate chips, rainbow sprinkles, and rainbow baking chips, if using.

Cover bowl with clingfilm/plastic wrap and refrigerate, at least one hour or overnight.

Preheat oven to 375° F / 190° C. Line a half sheet rimmed baking tray with parchment or a silicone baking mat.

With a 1 1/2-inch cookie scoop, scoop dough into 24 portions.

With your hands, roll each scoop into a ball. Arrange half of the balls on prepared baking tray. Refrigerate remaining dough until ready to bake.

Use your fingertips to flatten slightly. Press a few extra mini M&Ms on top of each ball.

Bake for 14-15 minutes, or until just very lightly golden brown around the edges. Do not overbake.

For perfectly round cookies, quickly reshape with a round metal cutter.

Cool on tray for 5 minutes, then transfer cookies to a wire rack to cool completely. Repeat with remaining dough.

Store peanut butter M&M cookies in an airtight container at room temperature for up to five days.