In a large bowl sift together 2 1/2 cups flour, 1 teaspoon baking soda, and 1 tsp salt.
In a separate bowl add 1 cup of softened unsalted butter, 1/3 cup of granulated sugar, and 1 cup of golden brown sugar. Using an electric hand mixer, mix it all together for about 1 minute. You want to make sure the sugar and butter are well incorporated together￼.
Now add 1 egg, 1 egg yolk, and 1 teaspoon of vanilla extract to the butter and sugar mixture. Mix again thoroughly.
Now add the flour mixture to the cookie batter and mix together thoroughly.
Add 1 1/4 chi semisweet chocolate chips to the cookie dough. Mix them in thoroughly.
Now line a 9-inch springform baking pan with parchment paper and press in the cookie dough. Now top it with extra semisweet chocolate chips and bake at 350°F for 22-25 minutes on the center rack.
In the meantime, make the chocolate buttercream by combining 1/2 cup unsalted softened butter, 1/2 cup cocoa powder, 2 1/2 cups powdered sugar, 1/2 teaspoon vanilla, and 2 tablespoons milk into a large bowl. Mix using an electric hand mixer for a smooth and creamy chocolate buttercream.
Once the cookie cake has cooled, transfer it to a large flat serving dish and pipe a border of frosting about the edges of the cake. I used a Wilton 2D piping tip. Then add on some rainbow-colored sprinkles and enjoy!