Preheat oven to 180º Celsius.
Sift plain flour, baking powder, baking soda, and salt into a large bowl. Set aside.
Place butter and caster sugar in a large bowl. Use an electric mixer to beat them together until light and fluffy. Add eggs, one at a time, beating well after each addition. Add flour in two batches, and beat until combined. Add vanilla extract and milk, and beat again until combined.
Line a 12-hole muffin tin with paper cases. Pour ¼ cup batter in each hole. Bake for 20-25 minutes, or until cooked through.
Turn them on to a wire rack. Leave to cool.