Preheat your oven to 350°F (175°C). Grease the cake pans and line the bottoms with parchment paper.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until well combined.
Add in the milk, vegetable oil, eggs, and vanilla extract. Beat the mixture on medium speed for 2 minutes until smooth and well incorporated.
Carefully stir in the boiling water. The batter will be thin but this is normal.
Gently fold in the rainbow sprinkles until evenly distributed.
Divide the batter evenly between the prepared cake pans.
Bake in the preheated oven for 30 minutes or until a toothpick inserted in the center comes out clean.
Remove the cakes from the oven, allow them to cool in the pans for 10 minutes, then transfer them to a cooling rack to cool completely.
Once the cakes are cooled, place one layer on a serving plate and spread chocolate frosting on top. Place the second cake layer on top and frost the top and sides of the entire cake.
Decorate the cake with additional rainbow sprinkles on top for a fun finish.