Preheat the oven to 350°F. Line the base of an 8x8-inch square metal baking pan with parchment paper leaving a 2-inch overhang at each side and secure the sides down on the edge of the pan with ¾-inch binder clips.

Make the crumb topping. Combine flour, brown sugar, cinnamon and salt in a medium bowl and mix together until evenly blended. Add the soft butter and blend it in using your finger tips until it reaches a crumbly and clumpy consistency. Cover the bowl and refrigerate it until needed.

Make the banana bread batter. Mash banana so it is quite smooth and measure out 1 cup (237ml). Place it in a bowl and add brown sugar and granulated sugar. Whisk until smooth. Add eggs and whisk until evenly incorporated. Add melted butter, sour cream and vanilla extract and whisk until combined and smooth.

Combine flour, baking powder, baking soda, cinnamon, nutmeg and salt in a medium bowl and whisk to blend evenly.

Add the dry ingredients and start folding them through gently. Before it is all incorporated and a few streaks remain, add chocolate chips and fold them in.

Assemble the cake. Spread the batter evenly into the prepared pan. Sprinkle the crumb topping mixture evenly over top and sprinkle a few extra chocolate chips over the crumble.

Bake for 38-42 minutes until a toothpick inserted into the middle comes out clean. Transfer pan to a wire rack, cover with foil and set aside to cool for at least 30 minutes.

Make the espresso glaze. Combine powdered sugar, espresso powder, cinnamon, salt and vanilla in a medium bowl. Gradually whisk in milk one teaspoon at a time until it is a drizzling consistency. Slice and enjoy!