Preheat oven to 375 degrees F (190 degrees C). Grease 12 muffin cups or line with paper muffin liners.
Combine 1 cup flour, oats, 1/3 cup brown sugar, and baking powder. Whisk together milk, peanut butter, banana, egg, oil, and vanilla. Add banana mixture to dry ingredients, mixing just until moistened. Fill prepared muffin cups 3/4 full.
In a small bowl, stir together 1/4 cup flour, melted butter and 2 tablespoons brown sugar. Sprinkle topping evenly over muffins.
Bake in preheated oven for 18 minutes, until golden brown. Serve warm.