Heat the oven to 175 C / 350 F. Grease a 23 x 23 cm / 9-inch square baking pan or line it with parchment paper.

Mash the bananas in a large mixing bowl until mostly smooth, with a few small lumps left.

Whisk in the melted butter, granulated sugar, brown sugar, egg, vanilla extract, and sour cream until smooth and pale yellow.

Sprinkle the baking soda, salt, and cinnamon over the wet mixture and whisk in until no streaks remain.

Add the flour and fold with a spatula just until no dry flour is visible. Stop as soon as it looks even, overmixing here is what makes banana snack cake turn dense and gummy instead of tender.

Pour the batter into the prepared pan and spread it evenly to the edges.

Stir the topping sugar and cinnamon together in a small bowl and sprinkle evenly over the batter.

Bake for 28 to 32 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs and the edges are golden brown.

Cool the cake in the pan for 15 minutes, then slice into 9 squares and serve.