Preheat oven to 350º. Lightly spray a 8x8 pan with cooking spray and line with parchment paper, leaving a 1" edge of paper above the pan for lifting cake out of pan. Lightly spray the paper.
For the batter: In a large bowl, mix together the dry batter ingredients: flour, sugar, brown sugar, baking powder, baking soda, salt and granola. Set aside.
For the streusel: In a small mixing bowl, mix the flour, brown sugar, spices and nuts or granola. Add melted butter and mix until crumbly. Set aside.
In a medium bowl, mix the mashed banana with the sour cream, oil, egg and vanilla. Add this to the flour mixture and stir until just combined.
Spread half of the batter into the prepared pan. Sprinkle with 1/2 of the streusel, breaking it up into medium sized clumps. Pour remaining batter over streusel and gently spread to edges using an off set spatula. Top with remaining streusel.
Bake for 30-35 minutes or until a toothpick inserted into center comes out clean.