Preheat your oven to 325°F and grease your baking pan. While the oven preheats, prepare the crumb topping by mixing together the flour, cinnamon, sugars, and cold butter cubes in a bowl. Use your fingertips to work the cold butter into the dry ingredients until the mixture resembles coarse breadcrumbs with pea-sized pieces of butter throughout—this creates that desirable texture. Transfer the crumb mixture to the refrigerator while you prepare the batter; the cold butter will help it stay chunky during baking.

In a separate bowl, whisk together the flour, baking soda, and salt. Set this dry mixture aside—this prevents lumps in your final batter and ensures even distribution of the leavening agent throughout the cake.

In a large bowl, whisk together the melted and cooled butter with both sugars and brown sugar until well combined. The mixture should be smooth and slightly emulsified. Add the egg, vanilla, and sour cream, stirring until fully incorporated. I like to use sour cream in banana bread because it adds a subtle tanginess and keeps the crumb incredibly moist—you'll notice the difference compared to recipes using just milk. Finally, fold in the mashed bananas until evenly distributed throughout the mixture.

Add the dry ingredient mixture from Step 2 to the wet ingredient base from Step 3, folding gently with a spatula until just combined. Don't overmix—a few small lumps are fine and will result in a more tender crumb. Overmixing develops gluten and makes the cake dense.

Pour half of the batter into your greased baking pan, spreading it evenly. Sprinkle the cinnamon sugar mixture (3 tbsp brown sugar mixed with 1.5 tsp cinnamon) over this layer. Pour the remaining batter on top, spreading gently to cover the cinnamon sugar layer without disturbing it too much. Finally, remove the crumb topping from the refrigerator and distribute it evenly over the entire surface of the batter. I find that letting the crumb topping stay cold until the last moment helps it maintain those desirable chunky pieces rather than melting into the batter.

Place the pan in the preheated 325°F oven and bake for 40-45 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter). The top should be golden brown. Let the cake cool in the pan for 30-40 minutes before adding the glaze—this allows it to set up enough to handle the liquid without falling apart.

While the cake cools, whisk together the sifted powdered sugar and milk until smooth and pourable. The sifting removes lumps and creates a silky glaze. Once the cake has cooled to warm (not hot), drizzle the glaze evenly over the top. It will set slightly as it cools, creating a light sweet finish that complements the moist crumb inside.