In a large bowl, combine the milk, eggs, sugar, vanilla, and cinnamon (if using). Add the oats to the liquid mixture and mix.
Let them sit in the bowl for 10 minutes to absorb the liquid.You can't skip this step. Melt the butter, and let it cool while the oats sit.
Give the oats in the bowl a quick mix. Add the melted butter, flour, baking powder, and salt. Mix to combine the batter, then let it sit for 5 more minutes.
Cook the pancakes:
Heat a non-stick pan or griddle over medium heat. Once hot, grease it and immediately pour up to ¼ cup batter per pancake. Always fill the scoop from the bottom of the batter up to pick up the oats that settle.
Cook the first side for 2 minutes, until the edges begin to look defined and bubbles form on the pancake. Flip the pancakes over and cook the second side for another 2 minutes. Transfer to a plate or a 200F oven to keep warm while you cook the rest.
Serve & Storage:
Serve warm, topped with your favorite pancake toppings and syrup.
Refrigerate leftover pancakes in an airtight container for up to 3 days. Reheat pancakes in the microwave for 30 seconds to 1 minute, in a pan, or in the toaster oven.