Preheat and prep: Heat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners. Set butter and eggs out if they’re not already at room temperature.

Whisk dry ingredients: In a bowl, whisk flour, baking powder, and salt. This keeps the cupcakes light and prevents clumps.

Cream butter and sugar: In a large bowl, beat butter and sugar on medium-high until pale and fluffy, about 2–3 minutes. Proper creaming helps the cupcakes rise well.

Add eggs and vanilla: Beat in the eggs one at a time, then add vanilla. Scrape the bowl so everything combines evenly.

Combine wet and dry: Add half the dry mixture, then the milk, then the rest of the dry mixture. Mix on low until just combined. Do not overmix or the cupcakes can turn dense.

Fill and bake: Divide batter evenly among liners, about two-thirds full. Bake 16–18 minutes, until the tops spring back or a toothpick comes out clean. Cool in the pan 5 minutes, then move to a rack to cool completely.

Make mermaid tails: Melt candy melts according to package directions. For a marbled look, drizzle a bit of each color into the mold and swirl gently with a toothpick. Tap the mold to remove air bubbles. Chill 10–15 minutes until firm, then pop out and set aside.

Whip the buttercream: Beat butter until smooth and creamy, 1–2 minutes. Add powdered sugar in stages, mixing on low at first to avoid a sugar cloud. Add vanilla, a pinch of salt, and 2 tablespoons cream. Beat on medium-high until fluffy, 2–3 minutes. Adjust with extra sugar for stiffness or a splash more cream for smooth piping.

Tint the frosting: Divide buttercream into 3–4 bowls. Tint each with gel colors—think teal, aqua, purple, and pink. Go light on color; you can always add more.

Create a swirl effect: Lay a piece of plastic wrap flat. Spread a stripe of each colored frosting side by side, forming a rectangle. Roll it into a log and twist the ends. Snip one end and place it, cut-side down, into a piping bag fitted with a large star tip. This gives you neat ocean swirls.

Pipe the cupcakes: Pipe tall swirls on completely cooled cupcakes. Start at the edge and spiral inward and upward. Cool cupcakes are key so the frosting holds its shape.

Decorate: Press a mermaid tail gently into the top at a slight angle. Add pearl sprinkles, a pinch of sanding sugar, and any shell candies. A light dust of edible glitter gives a sea-sparkle finish.

Set and serve: Let cupcakes sit 10–15 minutes so the frosting sets slightly. Arrange on a platter or cupcake tower for a party-ready centerpiece.