Preheat the oven to 350 degrees F (175 degrees C). Place a rack in the lower third of the oven. Spray a mini muffin pan with cooking spray. Combine water and flaxseed meal in a bowl and let sit 5 minutes. Meanwhile, whisk flour, oats, brown sugar, and salt together in a bowl and set aside. Mix thickened flaxseed, bananas, honey, yogurt, egg, applesauce, coconut oil, and vanilla extract together in a bowl. Stir flour mixture into the banana mixture. Fold in chocolate chips and walnuts. Use an ice cream scooper to scoop batter into the prepared muffin tin. Sprinkle cinnamon sugar on top. Bake in the preheated oven until a toothpick inserted in the center of a muffin comes out clean but without overbaking, about 18 minutes. Let muffins cool in the pan for 5 minutes. Transfer to a wire rack to cool completely. Freeze if desired.