Use a whisk or electric mixer to cream the butter and the sugar. Add the oil and repeat.
Add the eggs and beat until they become clearer in colour and have a fluffy texture. If you're using a stand mixer, remember to stop every once in a while to scrape the sides and base using a spatula.
Add the milk, baking powder, salt and cinnamon and give everything a good mix using a spatula. It will look split but don't worry, the flour saves it.
Add the flour and mix it in using a spatula until everything looks homogeneous.
Place the muffin cases inside you muffin pan.
Fill the cases leaving about 2mm without covering.