Preheat oven to 350°F (177°C). Line a 9-inch square pan or an 8-inch square pan with parchment paper, making sure to leave enough overhang on the sides. I use and prefer a 9-inch square pan. Set aside.

In a medium bowl, whisk together the flour, baking powder, baking soda, cornstarch, and salt. Set aside.

In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until light and fluffy, about 3 minutes. Scrape down the sides and bottom of the bowl as needed. Beat in egg, egg yolk, vanilla, and almond extract (if using). Scrape down the sides and bottom of the bowl as needed.

Add the dry ingredients to the bowl with the wet ingredients and beat on low speed until mostly incorporated, then increase to medium-high speed and beat until completely combined. The dough is thick and sticky. Finally, beat in the rainbow sprinkles.

Press the cookie dough evenly into the prepared baking pan as best you can. Use a spatula or lightly floured hands to press evenly; it will seem like a thin layer of dough.

Bake for 25–26 minutes or until very lightly browned around the edges. The center will still look very soft, that’s OK; the bars will continue to set as they cool. Do not over-bake.

Allow the bars to cool completely in the pan set on a cooling rack. The center will slightly sink—that’s expected.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cream, and vanilla extract and beat on low speed for 30 seconds, then increase to medium-high speed and beat for 3 full minutes. Stop the mixer and scrape down the sides of the bowl as needed. Add more confectioners’ sugar if frosting is too thin or more cream if frosting is too thick. Taste; beat in a pinch of salt if the frosting is too sweet. Beat in food coloring, if using.

Lift the cooled bars out of the pan by gripping the parchment paper overhang; transfer to a cutting board. Spread the frosting over the bars and decorate with extra sprinkles if desired. Cut into squares. Use a very sharp knife and wipe the knife between each cut for neat slices. Store bars in an airtight container at room temperature for up to 2 days or in the refrigerator for up to 5 days.