Preheat the oven to 350°F (175°C). Grease and flour an 8-inch square baking pan.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas until combined.
Add the flour mixture in three additions alternating with the sour cream and milk, beginning and ending with the flour mixture. Mix until just combined; do not overmix.
Pour the batter into the prepared baking pan and spread evenly.
In a small bowl, combine the light brown sugar, melted butter, and granulated sugar to create the snap topping mixture.
Sprinkle the snap topping evenly over the cake batter.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean and the topping is caramelized and crisp.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack before slicing and serving.