Preheat the oven to 400℉ and line a muffin tin with 12 paper liners.
In a small bowl, combine oats and milk. Leave to rest for 5 minutes.
In a large bowl, whisk together flour, baking powder, salt, and cinnamon.
Add maple syrup, butter, and egg to the oat mixture and combine.
Pour the oat mixture into the flour. Stir until just moistened.
Fold in half of the pecans.
Spoon the batter into a muffin tin. Sprinkle with reserved pecans. Bake for 15 to 18 minutes, until the muffin springs back when touched lightly with a finger and a toothpick inserted into the center of a muffin comes out clean.
Cool the muffins in the pan for 5 minutes. Remove the muffins from the pan and cool on a rack for approximately 30 minutes. Sprinkle with powdered sugar if desired.