Prep the pan and oven: Heat oven to 350°F (175°C). Grease an 8x8-inch or 9x9-inch baking dish, or line with parchment.
Whisk the wet ingredients: In a large bowl, whisk pumpkin puree, eggs, milk, maple syrup, melted coconut oil or butter, and vanilla until smooth.
Add the dry ingredients: Stir in oats, pumpkin pie spice, baking powder, baking soda, and salt.
Fold in any mix-ins.
Rest the batter: Let it sit for 5 minutes so the oats hydrate slightly. The mixture will thicken a bit.
Bake: Pour into the prepared dish and spread evenly. Bake 30–38 minutes, until the center is set and the top is lightly golden.
A toothpick should come out mostly clean with a few moist crumbs.
Cool and serve: Let cool at least 10 minutes. Slice and serve warm as is, or top with yogurt, a drizzle of maple syrup, or a few chopped nuts.