Whisk the milk, eggs, sugar, vanilla, and cinnamon (if using) in a large bowl, then stir in the oats. Make sure the oats are fully coated so they can soften evenly.

Leave the oat mixture undisturbed for 10 minutes, and melt the butter during that time so it has a chance to cool slightly. The oats should look plumper and less dry when the rest is complete.

Stir the soaked oats once, then fold in the cooled melted butter, flour, baking powder, and salt until no dry flour remains. Let the batter stand for 5 minutes until it thickens into a soft, spoonable consistency.

Warm a nonstick skillet or griddle over medium heat, grease the surface, and portion up to 1/4 cup batter for each pancake. Scoop from the bottom of the bowl so the heavier oats do not remain in a dense layer at the bottom.

Cook for 2 minutes, then turn the pancakes and cook for another 2 minutes. Flip when bubbles have formed and begun to pop, the edges look defined and matte, and the underside is golden.

Move cooked pancakes to a plate or hold them in a 200°F oven while finishing the remaining batter. A lightly pressed pancake should spring back gently instead of feeling wet in the center.

Serve the pancakes warm with syrup and any favorite pancake toppings. The warm surface absorbs syrup best while the edges are still lightly crisp.

Place cooled leftovers in an airtight container and refrigerate for up to 3 days. Warm pancakes in the microwave for 30 seconds to 1 minute, in a skillet, or in a toaster oven until heated through.