Preheat oven to 350° F. Spray the bottom and sides of an 8- or 9-inch square baking dish with cooking spray.
In a large bowl, whisk the eggs, milk, and vanilla. Add the oats, baking powder, cinnamon, and salt and mix well.
Pour mixture into prepared baking dish. Bake for 15 minutes. (It will go back in the oven with the topping).
While the oatmeal is baking during that first 15 minutes, prepare the pecan pie topping. In a small saucepan over medium-high heat, bring the butter, brown sugar, and maple syrup to a boil, whisking constantly to prevent the mixture from burning. Once it has thickened (after about 1 minute), stir in the pecans. Remove the pan from heat.
Remove the oatmeal from the oven (after the 10 minutes), spread the pecan mixture over the top, and return to the oven to bake for another 10-15 minutes, until the liquid has been absorbed, and the center is set.
Let cool for 10 to 15 minutes before serving alongside milk or yogurt. You can serve it warm or at room temperature. Cover and refrigerate leftovers for up to 4 days.