How to Make It
Preheat oven to 375°F. Process baking powder, salt, 1 1/2 cups of the flour, 1 cup of the Cheerios, and 1/2 teaspoon of the cinnamon in a food processor until powdery, about 10 seconds. Set aside.
Stir together eggs, vanilla, 1 cup of the brown sugar, and 3/4 cup of the butter in a large bowl until well combined. Add flour mixture; stir just until combined. Mash bananas with a fork in a medium bowl until mostly smooth. Gently fold mashed bananas and 3/4 cup of the walnuts into batter. Divide batter evenly among 15 lightly greased or paper baking cup-lined muffin cups.
Stir together remaining 1/2 cup Cheerios; 1/4 cup each of the flour, brown sugar, and walnuts; and 1/4 teaspoon cinnamon in a medium bowl. Stir in remaining 1/4 cup butter until combined. Sprinkle Cheerios mixture evenly over batter. Bake in preheated oven until a wooden pick inserted in center of muffins comes out clean, 18 to 20 minutes. Cool in muffin pans on a wire rack 5 minutes. Transfer muffins to a wire rack. Serve warm or cool completely.