Preheat the oven to 350 degrees and grease a 9-inch cake pan with nonstick cooking spray.
Whisk together the flour, baking soda, baking powder, cornstarch, and salt. Set aside.
In a separate bowl, beat together the butter, brown sugar, and white sugar until smooth and creamy.
Add the eggs and vanilla and beat until combined.
Add the dry ingredients and mix until blended.
Fold in 1 cup of chocolate chips.
Press the dough into the cake pan and top with remaining chocolate chips.
Bake for 20 to 25 minutes, until the edges are golden brown.
Remove from oven and allow to cool completely before removing from the pan.
To make the frosting, beat together the butter, cocoa powder, and vanilla extract until combined. Add the powdered sugar and 1 ½ tablespoons of heavy cream, then beat to combine. Add more cream, if needed, to reach the desired consistency.
Use a large star piping tip to decorate the edge of the cookie cake, then top the icing with rainbow sprinkles before serving.