Heat the oven to 400℉. Line a muffin tin with liners.
In a large bowl, whisk together the oat flour, brown sugar, baking powder, cinnamon, and salt. In a medium bowl, stir the milk, egg, and melted butter into the oatmeal, mixing well so that there are no lumps. Pour the liquid mixture into the dry ingredients. Stir in any add-ins if using.
Spoon batter into the muffin cups, filling each about 2/3rds full. Sprinkle with more brown sugar if you like. Let the batter rest while the oven heat, then place in the oven and bake for about 20 minutes, or until a tester comes out clean.