Preheat the oven to 175°C (350°F) and line a 12-hole muffin tin with cupcake cases.
Cream the butter and sugar together until pale and fluffy.
Add the eggs and vanilla, mixing until smooth.
In a separate bowl, whisk together the flour, baking powder, bicarbonate of soda, and salt.
Alternate adding the dry ingredients and milk/sour cream to the butter mixture, starting and ending with the dry ingredients. Mix gently until just combined.
Divide the batter evenly between cupcake cases (they should be about ¾ full).
Bake for 18–22 minutes, or until a skewer comes out clean. Leave to cool completely on a wire rack.