Step 1:
Preheat oven to 400°F (200°C). In a large bowl, mix together the flour, oats, and 1/2 cup sugar.

Step 2:
In a separate bowl, whisk the egg, melted butter, vanilla, and milk.

Step 3:
Add the wet ingredients to the dry ingredients and mix until combined.

Step 4:
Fill muffin cups about 2/3 full with batter. Bake for 12–14 minutes until golden.

Step 5:
While muffins bake, combine the remaining 1/3 cup sugar and cinnamon in a small bowl.

Step 6:
When muffins are out of the oven, brush the tops lightly with melted butter (optional) and sprinkle generously with the cinnamon sugar mixture.

Step 7:
Enjoy warm or at room temperature!