Preheat your oven to 180°c (160°c for fan-assisted oven or Gas Mark 4) and line a muffin tray(s) with 18 cupcake cases.
In a large bowl, mix the butter, sugar, flour and baking powder together until it resembles bread crumbs.
In a separate bowl, beat the eggs, milk and vanilla. Add half the wet ingredients to the dry ingredients and mix until combined. Continue to gradually add the rest of the wet ingredients to create a batter.
Add the sour cream right at the end, mixing to combine, before spooning the batter into your prepared cupcake cases, filling to 3/4 full.
Bake for around 20 minutes until a skewer inserted comes out clean.
Once cool enough to touch, transfer to a wire rack to cool completely.
For the Icing
Mix the butter and icing sugar together, gradually adding the milk and vanilla until light and fluffy.