Bake Cupcakes:

Prepare 24 cupcakes using the box mix, adding oil, water, and eggs per package directions; bake in paper liners (double-lined silver ones recommended to conceal chocolate cake color) at 350°F (175°C) for 19-23 minutes, dividing batter evenly.

Make Buttercream:

Cream butter and salt with an electric mixer until smooth. Gradually incorporate powdered sugar, 1/2 cup at a time, beating thoroughly after each addition. Once all sugar is added, pour in heavy cream one tablespoon at a time, then beat on high for about 30 seconds. Mix in vanilla until combined.

Tint Frostings:

Divide frosting: tint half turquoise (or your chosen fin/scales color), one-quarter purple (or another color), and leave the remaining quarter white (or your chosen color).

Fill Piping Bags:

Fill four piping bags: half of the turquoise in a bag fitted with a Wilton 2D tip; the remaining turquoise, purple, and white frostings each in separate bags with Wilton 1A (or larger) round tips. Have ready a small offset spatula, three small bowls, and a clean damp cloth.

Arrange Cupcakes:

Attach cupcakes to a full-size sheet cake board using double-sided tape, arranging them in a mermaid tail shape (see template).

Pipe Fins:

For the fins: using the 2D tip held slightly open, pipe continuously—first outlining the edge, then filling inward. Keep the bag well-filled to avoid air bubbles and restarts.

Pipe Scales:

For the scales: flip the board so fins are at the top. Pipe alternating colored dots by squeezing frosting directly onto each cupcake and lifting the bag straight up. Avoid adjacent same colors; fill gaps between cupcakes with extra frosting.

Create Scale Texture:

With the offset spatula, gently press down the dot tips and drag the spatula toward unfrosted cupcakes to create scale texture. Scrape excess into three separate bowls by color. Clean the spatula with the damp cloth between colors. To save time, do all dots of one color in a row, wipe, then the next color.

Pipe Waistband:

For the waistband: once scales reach near the top, pipe a V-shaped outline using any remaining color (white recommended if plentiful). Fill the center with extra frosting and smooth with the offset spatula. A slightly wet spatula helps eliminate bumps.

Decorate with Pearls:

Decorate the waistband and random scales with chocolate pearls. Slide the cake board onto the box bottom, then assemble the box around it. Store in the cake box until serving.