Preheat oven to 375°F.

In a large bowl or electric mixer fitted with a paddle attachment, cream together butter and sugar until smooth and fluffy. Add eggs, vanilla extract and milk, beat until smooth.

In a separate bowl sift together baking powder, salt and flour and add to wet ingredients. Blend until a sticky batter forms. Fold in blueberries.

Place paper cup cake cups into a muffin tin and spray lightly with pan release spray. Fill each cup 3/4 full with batter and sprinkle the tops with caster sugar.

Bake for 25-30 minutes or until muffins are golden brown and cooked completely. Let rest for 10 minutes before serving.