Lightly coat a 9 × 9-inch baking pan with nonstick cooking spray. Preheat the oven to 350°F while assembling the oatmeal mixture.
Place the old-fashioned oats, baking powder, ground cinnamon, and salt in a large mixing bowl. Stir thoroughly so the leavening and seasoning are distributed evenly among the oats.
In a separate smaller bowl, beat the egg until well blended. Add the milk, pumpkin puree, pure maple syrup, and vanilla extract. Whisk until the mixture becomes smooth, consistent, and free of visible streaks of egg.
Pour the pumpkin mixture over the prepared oat mixture. Stir until every portion of the dry ingredients is moistened and the mixture has an evenly combined, thick consistency.
Gently fold the semisweet chocolate chips through the oatmeal mixture, distributing them as uniformly as possible.
Transfer the mixture to the prepared 9 × 9-inch baking dish and spread it into an even layer.
Place the pan in the preheated oven and bake for approximately 30 minutes, or until the oatmeal is set and the surface has developed a golden-brown color.
Remove the baked oatmeal from the oven and allow it to rest for at least 5 minutes before slicing it into squares.