Step 1

Preheat the oven to 350 degrees F. Line two large baking sheets with parchment paper and set aside.

Step 2

Place soft butter in a large mixing bowl or in the bowl of a stand mixer fitted with the paddle attachment and beat until smooth and creamy. You can use electric beaters, your stand mixer or you can mix by hand with a sturdy spatula. Add both types of sugar and beat for about 2 minutes on medium high speed until pale and a bit fluffy. Stop to scrape down the sides of the bowl halfway through mixing.

Step 3

Add the egg and mix until evenly incorporated. Mix in egg yolk and vanilla. Scrape down the sides of the bowl as needed.

Step 4

Combine flour, baking soda, baking powder and salt in a separate medium bowl and whisk to blend evenly. Add the dry ingredients to the wet ingredients and fold them in gently until combined. Do not over-mix. Add the rainbow sprinkles and carefully fold them in with a spatula, being careful not to over mix the dough so that the sprinkles don’t break down nor bleed too much.

Step 5

Use a medium-sized cookie scoop to portion mounds of cookie dough and release them onto the prepared trays, spacing 2 to 3 inches apart. You can also grab portions of dough and roll them into balls with your hands. Flatten dough balls just slightly and bake for 10 to 13 minutes until lightly golden around the edges and the centers look slightly under baked. The cookies will continue to bake as they cool. Remove the pan from the oven and allow the cookies to cool on the cookie sheet for 3 minutes before transferring to a cooling rack to cool completely.