Preheat the oven to 180°C/350°F degrees. Line the 8x4 inch loaf pan with parchment paper.
Mix mashed banana, peanut butter, and honey in a large bowl. Add quick oats, protein powder, cinnamon, and a pinch of salt and mix until well combined.
Optional: Fold in add-ins of choice.
Transfer the dough to the pan. With the back of the spoon or a flat spatula press the dough so it is evenly flat.
Bake for about 20 min or until golden brown.
Remove from the oven and let it cool completely before serving. Cut vertically into 6 flapjack bars. Store in an air-tight container in a cold place or refrigerator for up to 5 days.