To begin, preheat your oven to 350 and line a 9x9 pan with parchment paper (or spray with non-stick spray).
To a medium mixing bowl, add your sugar and butter and cream them together with a hand mixer. Add your banana and egg and lightly blend them in. Add your milk and blend till smooth.
To the same bowl, add your flour and sprinkle your baking powder and salt across the top of it. Stir all of your ingredients together, to form the banana cake batter. Pour the batter into the pan and evenly spread it into the bottom.
To a small mixing bowl, add your sugar, flour and cinnamon and stir them together. Add your butter and mix your ingredients together.
Evenly sprinkle the mixture across the top of your coffee cake batter.
Bake the Banana Coffee Cake in the oven for 35-40 minutes, or till a toothpick inserted comes out clean.
Let cool, cut into pieces, and devour!
Stored in an airtight container in the fridge, this coffee cake should last about a week (but will taste best within the first 3-4 days).