Preheat the oven to 350°F (175°C) and grease your cake pan.
Mash the bananas in a bowl until creamy.
Cream softened butter and sugar together until fluffy in a separate bowl.
Beat in eggs, vanilla, and mashed bananas until smooth.
In a separate bowl, whisk gluten-free flour, baking powder, baking soda, and salt.
Alternate adding dry ingredients and milk to the wet mixture, stirring until just combined.
For the topping, combine gluten-free flour, brown sugar, cinnamon, and cold butter until crumbly.
Pour batter into the prepared pan and sprinkle with crumb topping.
Bake for 40 minutes, or until a toothpick comes out clean.
Cool for 10 minutes before slicing and serving.