Method:

Preheat oven to 350° F / 176° C. Line an 8 x 8 square tin with parchment. Set aside.

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

In a separate bowl or the bowl of a stand mixer, whip the butter on high speed until soft and fluffy.

Mix in the brown sugar, granulated sugar, egg, and vanilla bean paste, and continue mixing until well combined.

Add the dry ingredients and mix on low speed until just combined. Do not overmix.

With a rubber spatula or wooden spoon, fold in the chopped chocolate and rainbow sprinkles.

Transfer to prepared tin, using a rubber spatula to spread the batter evenly across the surface of the pan.

Bake in preheated oven for 30-35 minutes* or until golden brown, and a toothpick comes out mostly clean.

Transfer to a wire rack to cool completely. Once cool, prepare the frosting.

Chocolate Frosting:

Melt the butter in a small saucepan over medium heat.

Whisk in the cocoa powder, whole milk, and salt. Remove from heat.

Whisk in the confectioner’s sugar.

Pour frosting over cooled bar cookies.

With a rubber spatula or small stepped palette knife, spread the frosting, distributing it evenly across the top of the cookies.

Decorate with additional rainbow sprinkles.

Slice into squares and serve. Store leftover chocolate chip cookie bars in an airtight container at room temperature for up to three days.