In a large mixing bowl or the bowl of a stand mixer, mix butter and sugars on medium-high speed until light and fluffy.

Add peanut butter, egg, and vanilla extract to the bowl, and mix on high until well combined.

In a separate bowl, sieve flour, baking soda, baking powder, and salt. Gradually add dry ingredients to the butter mixture, mixing well after each addition.

Stir in mini M&Ms, chocolate chips, and rainbow baking chips, reserving a few of each for the tops of the cookies.

Wrap dough in clingfilm/plastic wrap and transfer to the refrigerator to chill, at least one hour.

Preheat oven to 375° F / 190° C. Line a baking tray with parchment or a silicone baking mat, and set aside until ready to use.

Remove chilled dough from refrigerator. Roll into golf ball-sized balls (you should end up with about 18 total). Roll balls in additional sugar, covering completely.

Place dough balls on prepared baking sheet, and use your fingers to flatten slightly. (Do not crowd your tray, as these cookies will spread! Use a second tray or bake in batches, if necessary.) Press a few extra chips, nuts, or candies into the top of each dough ball.

Bake cookies for 12-14 minutes, or until just lightly golden brown. Do not overbake.

Remove trays from oven and allow to sit for 5-10 minutes before transferring cookies to a wire rack to cool completely.

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