Prep your tools and oven. Line a 12-cup muffin tin with cupcake liners. Preheat the oven to 350°F (175°C). This helps the cupcakes rise evenly and prevents sticking.

Whisk the dry ingredients. In a bowl, whisk flour, baking powder, baking soda, and salt. Set aside so everything is ready to go once you start mixing the wet ingredients.

Cream the butter and sugar. In a larger bowl, beat butter and sugar on medium-high until light and fluffy, about 2–3 minutes. This step traps air for a soft, pillowy crumb.

Add eggs and vanilla. Beat in the eggs one at a time, then add vanilla. Scrape the bowl so everything mixes smoothly.

Combine wet and dry. Mix sour cream and milk together. Add the dry ingredients to the butter mixture in two additions, alternating with the milk mixture. Mix just until combined. Do not overmix or the cupcakes can turn dense.

Fill and bake. Divide the batter evenly among the liners, filling about two-thirds full. Bake 16–19 minutes, or until a toothpick comes out clean with a few moist crumbs. Let cool in the pan for 5 minutes, then transfer to a rack to cool completely.

Make the buttercream. Beat butter until smooth and creamy. Gradually add powdered sugar, then vanilla, salt, and 2 tablespoons of cream. Beat 2–3 minutes until fluffy. Add more cream if needed for a smooth, pipeable texture.

Color the frosting. Divide the buttercream into 3–4 bowls. Tint each with a small amount of gel food coloring. Start with a tiny drop; you can always add more for brighter tones.

Create the unicorn swirl. Lay a piece of plastic wrap on the counter. Spread a stripe of each color side by side to form a rectangle. Roll it into a log, then slide it into a piping bag fitted with a star tip. This gives you a multi-color swirl without fuss.

Decorate. Pipe tall swirls onto cooled cupcakes. Add sprinkles, a pinch of edible glitter, and, if you have them, unicorn horn and ear toppers. Aim for a playful, slightly imperfect look—this is part of the charm.