For the Cake:

1. Preheat the oven to 350°F and line a baking sheet with parchment paper. Place the unpeeled bananas on the baking sheet and bake for 15 minutes or until the skins are darkened and the bananas are very soft. They should smell caramely.
2. Remove from the oven and allow them to cool for a few minutes.
3. Meanwhile, start on your cake batter by creaming together the butter and sugar in the bowl of your stand mixer (alternatively, you can use a hand mixer). Mix until the butter and sugar are smooth, creamy, and lightened in color.
4. Carefully cut open the banana peels and scoop the softened flesh into the mixing bowl. I also add any juices that may have accumulated on the baking sheet – they have so much flavor. Mix until the bananas are incorporated.
5. Add in the eggs, sour cream, and vanilla and mix to combine.
6. In another bowl, whisk together the flour, cinnamon, nutmeg, baking powder, and salt.
7. Add in half of the dry ingredients to the mixing bowl and mix until just combined.
8. Pour in the milk and beat to combine.
9. Stir in the remaining dry ingredients and mix until just combined. Resist the urge to overmix this batter – it will get tough and dense, which is not what you want with coffee cake. Mix until there are no dry bits visible and then stop.

For the Streusel Layers:

1. In a bowl, combine the flour, sugars, cinnamon, and salt.
2. Add in the cold cubed butter and mix with your hands until the mixture is crumbly and the butter has been evenly dispersed. It will feel like slightly wet sand.

To Assemble:

1. Pour half of the batter into the bottom of a greased 9 x 13-inch baking pan. I also line my pan with parchment paper (with an overhang) so I can easily remove the coffee cake later. This is optional as long as your pan is well greased.
2. Sprinkle half of the streusel mixture over the first layer of batter.
3. Pour the remaining batter over top, spreading it as evenly as you can to cover the streusel without mixing it in.
4. Top with the remaining half of the streusel.
5. Transfer the pan to the oven and bake for 50-60 minutes or until a toothpick inserted into the center of the cake comes out with only moist crumbs (not wet batter).
6. Allow to cool before topping with the glaze.

For the Glaze:

1. Whisk together the powdered sugar, milk, and vanilla until smooth and creamy. Add more milk, as needed, to achieve a drizzling consistency.
2. Drizzle the glaze all over the top of the coffee cake, being as liberal as you like.

Slice the cake and serve.