Preheat the oven to 350F. Line a muffin/cupcake tray with liners and keep it ready.
In a mixing bowl add the mashed banana (use a fork to mash) followed by oil, vanilla extract and brown sugar.Mix well using a whisk.
In a separate bowl, add the dry ingredients like whole wheat flour, oats, baking powder, pinch of salt and baking soda. Mix everything well together.
Add dry mixture to the wet banana mixture. Combine everything using a spatula without over mixing.
Add chopped walnuts and milk. Gently fold the mixture so that walnuts and milk are nicely incorporated.
Scoop this batter into the pre-lined muffin tray. Fill each muffin liner till 3/4th full. Top it with some walnuts and oats (optional).
Bake at 350 for 25-30 mins or until done i.e. toothpick inserted in the center of muffin comes out clean without an batter stuck on it.
Let them cool in the tray for 10 mins and then transfer on to a cooling rack. Serve or store in airtight container for upto 3-4 days. Or freeze upto 2 weeks for later use.