In a medium mixing bowl, combine the flour, brown sugar, cinnamon, nutmeg, and salt. Add the cubed cold butter and work with your fingers to combine the butter into the dry ingredients. Keep going until the butter is completely combined and you have a dough that can be pressed into a ball. Pop it in the fridge, loosely covered until it's time to use.

Preheat the oven to 400 F while preparing the batter.

Mash the bananas well. Any chunks should be very tiny.

Melt the butter and add to the mashed bananas. Stir until the butter has completely melted.

Add the sugars, salt, vanilla, eggs, and sour cream to the butter/banana mixture. Whisk well to make sure it is fully combined.

Add the flour, baking powder, baking soda, cinnamon, nutmeg, and espresso powder and gently stir to combine, being careful not to over mix.

Pour half of the batter into a parchment lined loaf pan. Sprinkle half of the streusel over the batter.

Pour the rest of the batter over the streusel, smoothing the top into the corners and sides if needed. Sprinkle the rest of the streusel over the top.

Bake at 400 F for 10 minutes before dropping the heat to 350 F and baking for an additional 50-60 minutes.

Check the banana bread at 50 minutes with a cake tester or toothpick; it should not be wet. Keep baking until the cake tester comes out clean or with a few moist crumbs.

Cool on a wire rack for at least 15 minutes before trying to remove it from the pan.