Ingredients: For the muffins: 1/2 cup (1 stick) unsalted butter, melted 1/2 cup granulated sugar 1 large egg 1/2 teaspoon vanilla extract 1 cup all-purpose flour 1/2 teaspoon baking soda 3 overripe medium bananas, mashed (about 1 cup) 1 very ripe banana, for garnishing For the cinnamon su…