Heat the oven to 350°F (175°C). Put paper liners in six standard muffin cups. Leave the remaining wells empty.
Whisk flour, baking powder, and salt in a small bowl. Have the measured milk ready before starting the batter.
Beat cupcake butter and sugar for 2–3 minutes, until pale and fluffy. Scrape the bowl and beat in the whole egg and vanilla.
Mix in half the dry ingredients, then milk, then the remaining dry ingredients. Use low speed and avoid prolonged mixing.
Divide between six liners, filling about two-thirds full. Bake 18–22 minutes, until a tester comes out clean and tops spring back.
Let the cupcakes stand in the pan for 5 minutes, then move to a rack. Cool completely before decorating.
Beat frosting butter until creamy, then gradually add powdered sugar, vanilla, and 1 teaspoon milk. Add another teaspoon only if needed.
Spread frosting over the cooled cupcakes with a small spatula. For taller swirls, divide the frosting evenly before piping.
