Frosting: 170 g soft unsalted butter (1 1/2 sticks)
250 g powdered sugar (2 cups)
2 tbsp milk or cream
1 tsp vanilla and a pinch of salt
Optional: 2 tbsp seedless jam or fruit purée to flavor the frosting
How to make it
Prep (5 min): Heat the oven to 175C (350F) and line a 12-cup muffin tin.
Mix (10 min): Beat flour, sugar, baking powder, soda and salt with the soft butter on low until it looks like sand. Whisk the eggs, milk, sour cream and vanilla, and add in two goes, beating 30 seconds after each, until smooth.
Bake (20 min): Fill the cups two-thirds full and bake 18 to 20 minutes, until springy and a toothpick comes out clean. Cool completely.
Frosting (8 min): Beat the butter 3 minutes until pale. Add powdered sugar a bit at a time, then milk, vanilla and salt, and beat 3 minutes more until fluffy. Fold in jam if using.
Frost: Pipe or swirl generously with a spoon.
Tips
Room-temperature eggs and milk keep the batter from curdling.
Fruit purée in the frosting, Violet-style, cuts the sweetness.