First of all, start preheating oven to 350F or 180C.
Mix milk and vinegar together in a bowl and keep it aside till we need it.

Mash banana with the help of a fork and keep it ready.

In a small bowl add 1 tsp of flour and choco chips. Mix properly so that all choco chips are covered with wheat flour. This prevents them from drowning in the batter.

Take a mixing bowl and sift flour, baking soda, cinnamon, and nutmeg powder. If you do not like adding cinnamon and nutmeg powder just skip it. But I will urge you to add them and see the magical flavor they give to banana muffins.

In another mixing bowl take the mashed banana, sugar, mixture of milk-vinegar, oil/butter, and vanilla essence. Whisk nicely.

Now add dry ingredients in wet ingredients mixing bowl and combine them until you do not see any particle of flour.

Add choco chips in this batter and fold them gently.
Line the muffin liners in pan and pour this batter equally. This batter will give 12 muffins. Spread walnuts on top generously.

Bake for 15 minutes. Check by inserting a toothpick. If it comes out clean then muffins are ready.
Cool them on a cooling rack before serving.