Preheat the oven to 400°F. Lightly grease a standard muffin pan or line the pan with papers and grease the papers., In a large bowl, stir together the flour, oats, sugar, salt, and baking powder., In a separate bowl, beat together the milk, butter (or oil), eggs, and vanilla., Stir the wet ingredients into the dry, mixing only enough to combine evenly., Scoop the batter into the prepared pan. Sprinkle with sugar., Bake for 18 to 20 minutes, until golden brown and a cake tester inserted in the center comes out clean. Remove from the oven and cool on a rack for 5 minutes before tipping the muffins out of the pan and returning to the rack to finish cooling completely., Store, wrapped, at room temperature for 2 days or freeze for longer storage.