Preheat the oven to 350℉ (175℃).

In a large bowl, use an electric mixer to cream together ¾ cup of softened butter, ½ cup of granulated sugar, and ½ cup of brown sugar until well combined and fluffy.

Beat in 2 eggs and 1½ teaspoons of pure vanilla extract until fully incorporated into the mixture.

In a separate bowl, whisk together 2½ cups of all-purpose flour, 1 teaspoon of baking soda, and ½ teaspoon of salt.

Gradually add the flour mixture to the wet ingredients, mixing lightly after each addition until the dough is just combined.

Fold in 1¼ cups of chocolate chips into the dough until evenly distributed.

Press the dough evenly into a 9-inch round cake pan, smoothing the surface.

Bake in the preheated oven for 15-20 minutes or until the edges are golden brown.

Remove the cookie cake from the oven and let it cool in the pan.

In a large bowl, beat together ¼ cup of softened butter and 1 cup of powdered sugar until combined.

Mix in ⅓ cup of unsweetened cocoa powder, 1 teaspoon of pure vanilla extract, and 2 tablespoons of milk. Beat the mixture until smooth.

Gradually add the remaining 1 cup of powdered sugar ½ cup at a time, beating until smooth after each addition. If necessary, add 1-2 additional tablespoons of milk to reach the desired frosting consistency.

Once the cookie cake has cooled completely, frost it by piping designs or spreading the chocolate buttercream frosting over the top.

Top the frosted cookie cake with colored sprinkles for decoration.