Preheat the oven to 350℉. Spray an 8-inch round cake pan with non-stick cooking spray and set aside.
Add the softened butter and brown sugar to the bowl of a stand mixer. Beat on a high speed using a flat beater attachment for 3 minutes, or until pale and creamy.
Add the eggs and vanilla extract to the butter mixture and beat on medium speed until well combined, scraping the sides of the bowl with a spatula as necessary.
Add the flour, cornstarch, baking soda and salt to the bowl and beat on a low speed until just combined. Stir through 1 cup of the chocolate chips.
Transfer the mixture to the prepared cake pan and smooth the surface. Evenly scatter over the remaining ¼ cup of chocolate chips.
Bake the cookie cake for 20 minutes, or until just turning golden in color and just cooked through. Allow to cool in the pan completely.
To make the vanilla buttercream, add the softened butter, vanilla extract, and salt to the bowl of a stand mixer. Beat on high speed using a wire whisk attachment for 2 minutes or until pale and creamy. Add the powdered sugar and beat on a low speed, increasing to high speed, for 2 minutes or until pale and fluffy.
Pipe the buttercream around the edge of the cookie cake, and decorate with sprinkles, if using.