In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, granulated sugar, and brown sugar on medium speed for 3–4 minutes until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Mix in the vanilla extract and Amoretti Birthday Cake Flavoring.

In a separate bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt.

Slowly add the dry ingredients to the butter mixture, mixing just until combined. Avoid overmixing.

Fold in the rainbow jimmies and white chocolate chips, if using.

Cover and chill the dough for at least 30 minutes.

Preheat your oven to 350°F. Line baking sheets with parchment paper.

Scoop the dough using a large cookie scoop (about 3 tablespoons each) and place the dough balls about 3 inches apart.

Gently flatten each dough ball just slightly with your palm. Press a few extra sprinkles onto the tops.

Bake for 10–12 minutes, until the edges are lightly golden but the centers still look slightly underdone.

As soon as the cookies come out of the oven, use a large round cookie cutter or drinking glass to gently swirl around each cookie to create perfectly round edges.

Let the cookies cool on the baking sheet for 5 minutes before transferring to a cooling rack.