Preheat oven to 350 degrees. Grease a 9x9 square pan with non-stick spray.

Add butter to a mixing bowl and beat on high with stand-mixer or hand-mixer until light and fluffy (about 1 minute). Add brown sugars and mix on medium-high until smooth (about 1 minute). Mix in egg on medium until completely combined. Add egg yolk and vanilla and beat on medium speed until completely combined.

In a separate bowl, whisk flour, cornstarch, baking soda, and salt until combined. Add flour mixture to wet ingredients about 1/2 cup at a time. Mix until smooth. Add 1 1/4 cup of chocolate chips (the rest will be added later) and mix until combined (5 seconds).

Using a spatula, scoop all dough into prepared pan. Spread evenly, making sure to completely cover the entire bottom of the pan. Bake in preheated oven for 20-25 minutes. The edges will begin to darken around 15 minutes. Add foil around the edges to keep them from crisping too much. The cookie will be done when center no longer jiggles. You can also insert a toothpick to check if it's done. I like my bites slightly doughy so I take them out when the toothpick is slightly dirty. Add remaining chocolate chips as soon as you remove from oven.

Let pan cool for 30 minutes and then place in fridge to cool for another 1 1/2 hours. Do not cut until the cookie cake is COMPLETELY cool.

Once the cookie cake is completely cool, it's time to cut them into little squares. Using a sharp knife, release cake from the edges of the pan. Use a pancake flipper to help release cookie from the pan. Be very gentle so you don't break the cake. Place whole cookie on cutting board. Using a very sharp knife, cut cookie into 8 rows and 8 columns. This will give you 64 bites. Now, they are ready for frosting. See below for instructions on preparing frosting.

Measure cocoa and, if lumpy, sift it. Add all frosting ingredients to the bowl fitted for a stand mixer. Use whisk attachment to stir ingredients on low until completely combined. Scrape down sides of bowl, if needed. Increase speed to medium-high and beat until smooth (about 1 minute). Let cool at room temperature while the cookie cools (at least 30 minutes).

When ready to frost cookie bites, place a star tip in the bottom of a disposable piping bag. Cut tip of bag so tip slides right in but doesn't fall through. Fill bag about 1/2 full with fudge frosting. Twist end of bag to keep frosting in bag and help push it out.

Using two hands, squeeze a little frosting onto each cookie bite, about 1/2 teaspoon. Immediately decorate with sprinkles or mini chocolate chips. Store bites at room temperature for up to 4 days (if they last that long!).