Preheat oven to 325° F. Line an 8x8-inch glass or ceramic baking dish with parchment. Alternatively, for a metal pan, heat the oven to 350° F and prepare the pan in a similar manner.

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.

Pour the batter into the prepared pan. Scatter the topping, in clumps, on top of the batter.

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.