Preheat your oven to 175°C (350°F). Grease two 9-inch round cake pans and line the bottoms with parchment paper.

In a large bowl, sift together flour, cocoa powder, sugar, baking powder, baking soda, and salt.

In a medium bowl, whisk together eggs, milk, oil, and vanilla extract until smooth.

Gradually add the wet ingredients to the dry mixture. Mix on low speed with a hand mixer or stand mixer until just combined.

Carefully add boiling water and mix until the batter is smooth (batter will be thin). Gently fold in the rainbow sprinkles with a spatula.

Divide the batter evenly between the prepared pans. Tap pans gently on the counter to remove air bubbles.

Bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes, then turn out onto a wire rack to cool completely.

For the buttercream, beat butter on medium speed until creamy. Add powdered sugar, cocoa powder, salt, and vanilla. Mix on low to combine, then increase to medium-speed, adding cream or milk a bit at a time until a smooth, fluffy consistency is reached.

Once cakes are completely cool, place one layer on a serving plate. Spread evenly with frosting. Top with the second cake layer, and frost the top and sides. Scatter more sprinkles over the top and sides as desired.