To make the topping, whisk together flour, brown sugar, sugar, cinnamon, and salt. Stir in melted butter with a fork until moist clumps form. Set aside the topping.
For the cake, whisk together flour, buttermilk powder, baking powder, baking soda, and salt in a medium bowl.
In a large mixing bowl, cream the butter and sugar together. Add eggs, bananas, milk and vanilla and mix until well combined. Mix in the dry ingredients just until blended.
Bake 55 to 60 minutes until a toothpick inserted in the center comes out clean.
To make the icing, whisk together powdered sugar, water and vanilla until smooth. Drizzle over warm cake.