Combine everything except for the butter and oil into a large measuring jug. Use a stick blender to blend until it makes a silky smooth batter.
Let the batter rest in the fridge for a minimum of 10 minutes, preferably 20.
While the batter is resting, put a 9 inch cast iron skillet in the oven and preheat to 450. If you don't have a cast iron, any oven-safe dish will work.
When the batter has rested and the oven and pan are hot, remove the pan from the oven.
Add the oil and butter to the pan and give it a swirl till the butter melts.
Pour the batter into the hot skillet, and get it back in the oven to bake for 20 minutes and watch that pancake rise.