Preheat the oven to 400 F (204 C). Line a 12 ct. muffin pan with cupcake liners. Make the spiced streusel by whisking the sugar and flour together in a large bowl until well combined. Work in the cold, cubed butter until well combined in the texture of wet sand.
In a larger mixing bowl, whisk the mashed banana, butter, sugar, eggs, sour cream and vanilla extract together until smooth and well combined. The liquid will be a pale yellow. Stir in the dry ingredients with a spoon until just combined.