Preheat the oven to 175°C and grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt for the cake.

Add the egg, milk, vegetable oil, and vanilla extract. Beat on medium speed until well combined.

Gradually stir in the boiling water until the batter is thin and smooth.

Pour the batter evenly into the prepared cake pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then remove and transfer to wire racks to cool completely.

For the crumble, combine flour, brown sugar, unsalted butter, and a pinch of salt in a bowl. Mix until the mixture forms coarse crumbs.

Spread the crumble mixture on a baking sheet and bake at 175°C for 10-12 minutes, until golden brown. Let cool.

Once the cakes are cool, spread a layer of frosting (if desired) on the first cake and sprinkle some of the crumble over it.

Place the second cake layer on top and frost the entire cake if desired.

Decorate with rainbow sprinkles all over the cake.

Garnish with edible mermaid tails and seashells for a whimsical finish.