Step 1: Prepare the Equipment

1. Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it with nonstick spray.
2. Mash the bananas in a medium bowl until smooth.

Step 2: Mix the Dry Ingredients

1. In a large mixing bowl, whisk together the flour, baking soda, salt, and cinnamon (if using). Set aside.

Step 3: Combine the Wet Ingredients

1. In another bowl, beat the peanut butter, granulated sugar, and brown sugar together until smooth and creamy.
2. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract, mashed bananas, oil, and milk until fully incorporated.

Step 4: Combine Wet and Dry Ingredients

1. Gradually add the wet ingredients to the dry ingredients, folding gently with a spatula until just combined. Be careful not to overmix, as this can lead to dense muffins.
2. Fold in any optional add-ins, such as chocolate chips or nuts, if desired.

Step 5: Fill the Muffin Tin

1. Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.

Step 6: Add the Topping

1. In a small bowl, mix the granulated sugar and cinnamon for the topping. Sprinkle it evenly over the muffin batter.

Step 7: Bake

1. Bake the muffins in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
2. Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.