Preheat your oven to 350°F (175°C) and prepare a 10-inch loaf pan.

In a bowl, whisk together the dry ingredients: flour, baking powder, cinnamon, nutmeg, and salt.

Mash the overripe bananas in a separate bowl until smooth with a few chunks.

Cream butter with granulated and brown sugars until fluffy.

Incorporate eggs and vanilla into the butter-sugar mixture, then fold in mashed bananas and sour cream.

Gradually mix the dry ingredients into the wet ingredients until just combined.

Pour the batter into the prepared loaf pan, smoothing the top.

For the streusel, mix flour, sugar, and cinnamon; cut in cold butter until crumbly.

Sprinkle streusel topping over the banana bread batter.

Bake for 50 to 60 minutes, checking for doneness with a toothpick.

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.