Take the butter and eggs out of the fridge an hour ahead. Heat the oven to 170°C (150°C fan) and line a 12-hole muffin tin with paper cases.
Beat the butter and sugar for 3–4 minutes until pale and fluffy.
Add the eggs **one at a time**, beating well after each. Add the vanilla.
Sift in the flour, baking powder and salt, add the milk, and fold together **just until the flour disappears.** Don’t beat it.
Divide between the cases, filling each **two-thirds full** — no more.
Bake for 18–20 minutes, until risen, level and golden, and a skewer comes out clean. Don’t open the oven in the first 15 minutes.
Cool in the tin for 5 minutes, then move to a rack to cool **completely** before icing.
For the buttercream, beat the butter alone for 2 minutes, then add the sifted icing sugar in three additions, beating well. Add the milk, vanilla and salt and **beat for a further 5 minutes** until pale and light.