Make the cookie cake: Preheat the oven to 350°F and grease a round 9" pan (we also like placing parchment paper on the bottom).
In a small bowl, whisk together the flour, baking soda, salt, and cornstarch.
In separate large bowl or the bowl of a stand mixer fitted with the paddle attachment, add the butter, brown sugar, and sugar. Beat with a hand mixer or the stand mixer until light and fluffy. Add the milk and vanilla and beat until creamy.
Add the dry mixture to the wet mixture and mix until just combined. Fold in the chocolate chips.
Press the dough evenly into the prepared pan. Sprinkle desired amount of extra chocolate chips on top and gently press into the dough.
Bake for 14-16 minutes, until lightly golden-brown. Allow to cool, then transfer to a serving platter (removing parchment paper if necessary).
Make the buttercream: In a medium-sized mixing bowl or bowl of a stand mixer, beat the butter and powdered sugar until light and creamy. Add the milk, vanilla, and cocoa powder (if using) and beat until creamy. Add more milk, as needed, to reach a creamy but firm consistency.
Decorate: Place the frosting Into a piping bag fitted with a tip of choice. Pipe around the edges. Top with sprinkles, if desired. Serve by cutting into desired amount of slices.