Start by heating your oven to 350°F (175°C). Prepare your baking pan by either greasing it with butter or lining it with parchment paper to stop the bread from sticking.
Next, get the crumb topping ready. In a small bowl, mix together brown sugar, flour, and cinnamon. Add cold butter cut into small pieces, then use a pastry cutter or fork to blend the butter into the dry ingredients until the mixture looks crumbly. Put this bowl in the fridge to keep it cold while you make the batter.
For the batter, take a large mixing bowl and mash the ripe bananas thoroughly with a fork or potato masher until smooth. Add softened butter, sugar, and either sour cream or milk, mixing everything well. Break the eggs in one at a time, beating the mixture after each addition to fully combine them.
Add vanilla extract and cinnamon to the wet ingredients. In a separate bowl, sift together the flour, baking powder, baking soda (if you’re using it), and salt. Slowly add these dry ingredients to the banana mixture, folding them in gently. Don’t over mix just combine until you don’t see any more dry flour. Over mixing can make the bread dense.
Pour the batter evenly into your prepared pan. Take the crumb topping out of the fridge and sprinkle it generously over the top, covering the batter.
Bake the bread in the oven for 50 to 60 minutes. Check if it’s done by sticking a toothpick in the center if it comes out mostly clean with just a few moist crumbs, it’s ready. If the toothpick comes out wet, give it another 5 to 10 minutes and check again.
Once baked, take the bread out and let it cool on a wire rack for at least 15 minutes before slicing. This resting time lets the bread set properly and improves its texture.