Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan with cooking spray and line the bottom with parchment paper. Set it aside.
In a medium bowl,combine flour, baking soda, and salt. Mix well, and set aside.
In a separate bowl, beat the butter, brown sugar, and granulated sugar for about 3-5 minutes until light.
Add the eggs one at a time to the butter mixture and beat for 3 minutes until pale and fluffy. Add the vanilla extract.
Slowly add the flour mixture to the wet mixture, combining until everything is well incorporated. Stir in the chocolate chips.
Transfer the batter to the prepared pan and smooth the top with a spatula. Sprinkle the remaining 2 tablespoons of chocolate chips over the top.
Bake for 23–25 minutes, or until the cookie cake turns a light golden brown color.
Once done, remove the cookie cake from the oven and let it cool for 10 minutes. Run a butter knife around the sides of the cake, then slowly release the springform pan and transfer it to a cooling rack.
Meanwhile, prepare the chocolate buttercream by beating butter, cocoa powder, milk, and vanilla extract using a hand mixer. Slowly add the powdered sugar and beat until smooth and creamy for 3-5 minutes. Transfer the chocolate buttercream to a piping bag with a star nozzle.
Pipe the chocolate buttercream around the edge of the cookie cake and sprinkle rainbow sprinkles on top.
Once done, slice the chocolate chip cookie cake and serve.