Beat the butter in the bowl of an electric mixer until light and creamy. Beat in the vanilla, Natvia and eggs one at a time until combined. Sift over the flour and baking powder and whisk until you have a silky batter. Add 1 tbs milk at a time if the batter is too thick, until you get the silky consistency.
Divide batter between a 12 x hole cupcake greased tin.
Bake for 15-20 mins, until a skewer inserted in the middles comes out clean, without any crumbs or gooey batter on it. Stand for 10 minutes in the tin before removing to a wire rack to cool completely.