Preheat the oven to 375 degrees Fahrenheit.
Butter an 8 x 8″ or 9 x 9″ pan or 7 x 11 small casserole dish.
In a medium bowl, mix together the rolled oats, pecan pieces, baking powder, cinnamon, allspice, and kosher salt.
Dump the dries into the prepared pan. Add the diced apple in a layer on top.
In the same bowl, whisk the egg, then add the milk, maple syrup, and vanilla. Drizzle the milk mixture over the oats. Use a fork to lightly stir so it gets evenly incorporated.
Bake 40 to 45 minutes until the top is nicely golden and the oat mixture has set.
Remove from the oven and allow to cool for at least 10 minutes.
Before serving, melt the butter with the remaining 1 tablespoon maple syrup. Drizzle the butter evenly over the top and serve.
Store leftovers refrigerated for up to 1 week: reheat in a 300 degree oven or microwave until warm.
