Pre-heat oven to 200C (180 fan, gas mark 6). Put milk, oil, vanilla essence, and egg (egg last) in jug and stir.
Put flour, sugar, and baking powder in a bowl and make a well.
Pour wet ingredients into well, and mix until porridge consistency.
Half fill the muffin cases, and put some of your choice of filling (chocolate chips etc.) on top of the mixture. Pour some more mixture on top of that, and then sprinkle the rest of the filling on top. Bake in oven for 15-20 minutes until golden.