Milk+Vinegar: I have combined both to make homemade buttermilk. If you have store-bought buttermilk ready in your fridge then use that. Make sure milk or buttermilk is at room temperature.
Oil: I recommend using flavorless oil like corn, canola, vegetable, safflower, etc.
Variations: This is a very versatile muffin batter. Skip chocolate chips and topping, you will have the nice vanilla-flavored batter ready. Customize however you like (a few ideas, add dried fruits, nuts, berries, raisins, dates).
Don’t Over-mix: When combined dry and wet ingredients, do not over mix. This is very important to make fluffy and soft muffins. If you overmix, you will get dense, tough muffins.
This is the thick muffin batter, do not tempted to any other liquid into the batter.
Make vegan muffins: substitute the regular milk with any vegan milk and use vegan chocolate chips. Yes, I have tried myself and it works.