Preheat oven to 350 degrees F. Spray a 9x13 baking pan with nonstick spray.
To make the crumble topping, melt butter and set aside to cool for 2-3 minutes. Stir in brown sugar and granulated sugar. Add flour, salt, and cinnamon and stir to combine. Mix in chopped walnuts, use a rubber spatula or back of fork to crumble the topping together. Set aside.
In a mixing bowl, whisk together the flour, baking powder, cinnamon, and salt. In the bowl of an electric mixer, beat butter and sugar together until blended. Add mashed bananas and beat until just combined. Add eggs and vanilla and beat until combined. Stir in the flour and milk alternately, beginning and ending with the milk. Pour half of the cake batter into the baking pan. Sprinkle half of crumb topping. Then pour the remaining batter on top. Finish with the remaining crumble topping. Bake in the preheated oven for 50-60 minutes or until a toothpick inserted in the center comes out clean.
TIP: Cover cake with aluminum if top begins browning too much. Allow to cool before glazing.
To make glaze, whisk together powdered sugar with vanilla extract and milk. Add more powdered sugar if you want a thicker glaze or more milk for a runnier glaze. Drizzle on top of cake.