Prep the pan and oven. Heat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy lifting, or lightly grease the pan.
Whisk the wet ingredients. In a large mixing bowl, whisk together 2 large eggs, 1 3/4 cups milk, 1/2 cup maple syrup or honey, 2 teaspoons vanilla, and 1/2 cup creamy peanut butter until smooth.
If your peanut butter is very thick, warm it briefly so it blends easily.
Add the dry ingredients. Stir in 3 cups old-fashioned rolled oats, 1 1/2 teaspoons baking powder, 1/2 teaspoon salt, and 1 teaspoon cinnamon (if using). Mix until the oats are evenly coated and the batter is combined.
Fold in chocolate chips. Add 1/2 to 3/4 cup chocolate chips. Fold gently so they’re evenly distributed.
If you like, reserve a small handful to sprinkle on top.
Rest the batter. Let the mixture sit for 5 minutes so the oats begin to hydrate. This helps the bars bake up cohesive, not crumbly.
Fill the pan. Spread the batter into the prepared pan, smoothing the top. Sprinkle reserved chocolate chips on top for a bakery-style finish.
Bake. Bake for 28–35 minutes, until the edges are set and lightly golden and the center no longer looks wet.
A toothpick inserted near the center should come out mostly clean with a few moist crumbs.
Cool and slice. Let the pan cool on a rack for at least 20–30 minutes. Use the parchment overhang to lift out, then slice into 9–12 bars.
Serve. Enjoy warm, room temp, or chilled. For a cozy touch, drizzle with a little warm peanut butter or a splash of milk.