In a large bowl, whisk together the buttermilk, discard, oil, egg, and vanilla extract.
Add the flour, sugar, and salt, and whisk until just combined. Don't worry if it looks a little lumpy.
Cover the bowl (with a damp tea towel or something like a shower cap) and set it in the refrigerator overnight, or for about 8 hours.
Day 2
The following morning, the batter will likely have separated a little bit, and have a watery looking layer on the surface. This is normal.
Add the baking powder and baking soda, and whisk until fully combined. Set the batter aside for 10 minutes.
While the batter is resting, heat a large frying pan over medium heat. Make sure the pan is hot before starting to cook the pancakes.
Lightly grease the bottom of the pan with butter or oil, then add scant ¼ cup-fulls of batter for each pancake. Cook for about 30 seconds, or until bubbles start to appear, then flip and cook about another 30 seconds.
Repeat until all of the batter has been used, keeping pancakes covered in a warm oven before serving.