Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.

Make the Cake Batter: In a large mixing bowl, combine flour, sugar, baking powder, and salt. Add softened butter, milk, and vanilla extract. Mix until smooth. Beat in eggs one at a time until fully incorporated. Fold in 1 cup of rainbow sprinkles gently.

Bake the Cakes: Divide the batter evenly among the prepared cake pans. Bake in preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out clean. Let cakes cool in pans for 10 minutes, then remove to wire racks to cool completely.

Assemble the Cake: Once the cakes are completely cooled, layer as follows: Place the first cake layer on a serving plate. Spread a layer of whipped cream and sprinkle some crushed chocolate cookies. Place the second cake layer on top and repeat the process. Top with the third cake layer.

Frost the Cake: Spread whipped cream over the top and sides of the assembled cake. Decorate with additional rainbow sprinkles, whole chocolate sandwich cookies, and any other colorful candies or sprinkles of choice.