Prepare the oven and pan: Preheat the oven to 350°F (175°C). Line a standard 12-cup muffin pan with paper liners. If you are not using liners, lightly grease the cups.

Mash the bananas: Peel the bananas and place them in a large mixing bowl. Mash with a fork until mostly smooth, leaving a few small pieces for texture. You should have about 1 1/4 cups (285 g) mashed banana.

Mix the wet ingredients: Add the eggs, Greek yogurt, maple syrup, melted butter, and vanilla to the mashed bananas. Whisk until the mixture is smooth and evenly combined.

Combine the dry ingredients: In a separate bowl, whisk together the flour, protein powder, baking powder, baking soda, cinnamon, and kosher salt. Break up any visible lumps of protein powder.

Combine the batter: Add the dry mixture to the banana mixture. Fold gently with a rubber spatula just until no large streaks of dry flour remain. Do not overmix; the batter should be thick and scoopable.

Fill the muffin cups: Divide the batter evenly among the 12 prepared muffin cups, filling each about three-quarters full. If using the optional walnuts or mini chocolate chips, sprinkle them evenly over the tops.

Bake the muffins: Bake at 350°F (175°C) for 17 to 20 minutes. The tops should be lightly golden and spring back gently when pressed in the center.

Check for doneness: Insert a toothpick into the center of one muffin. It should come out with a few moist crumbs but no wet batter. If the centers are still wet, bake for another 2 to 3 minutes and check again.

Cool before serving: Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack for another 5 minutes. The crumb will become firmer as the muffins cool.