Preheat your oven to 350°F (177°C). Line a cupcake pan with 12 cupcake liners and set aside.
In another bowl, mix the flour, baking powder, and salt until well blended.
In a large bowl, beat the butter and sugar together with a stand mixer (or by hand) until the mixture is light, pale, and fluffy.
Add the dry ingredients into the butter mixture in three parts, alternating with milk and adding in the sour cream. Mix gently between each addition until just combined.
Use a large batter scoop to divide the cupcake batter evenly into the 12 prepared liners.
Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean.
Remove cupcakes from the oven and let them cool completely on a wire cooling rack before frosting.
Buttercream Frosting
Add butter to a clean mixing bowl and beat for 5 minutes, until light and doubled in volume.
Add vanilla, then gradually add powdered sugar and milk, 1/2 cup at a time, beating well between additions.
Add your favorite food coloring (blue, lavender, teal) and continue beating for 8–10 minutes, until smooth and fluffy.
Fill large piping bags using a spatula or spoon. Use jumbo round or French piping tips to frost cupcakes.
Mermaid Tails and Shells
Place candy melts in squeeze bottles and set them in a saucepan of medium heat water (not boiling). Cover and let them melt on medium-low.
Once melted, invert bottles into a cup and let cool slightly until the chocolate settles to the tip.
Squeeze candy melt into mermaid tail molds, smooth the surface with a cake spatula, and place the tray in the fridge for 10 minutes to set.
Remove the set candy tails from the molds and place them on a baking sheet. Use a food-safe brush to apply edible glitter dust.
Once cupcakes are frosted, gently press a mermaid tail into each one. Finish with mermaid sprinkles for a magical touch.
Notes
To thin out the icing, add in the milk slowly and gradually.
When done with the candy melt, place the bottle on its side in the freezer for 5 minutes. When solid, just squeeze to bottle to break up all of the solidified candy, and empty all of the candy into a plastic bag.
You can make the chocolate tails ahead of the cupcakes, and store them in a plastic bag in the fridge.