Preheat the oven to 350 F (177 C). Lightly grease and line the bottom and long sides of a 9 x 9 inch (23 cm) cake pan with parchment paper.
In a large bowl, whisk together the flour, brown sugar, granulated sugar and ground cinnamon for the streusel topping. Work in the melted butter a little at a time until a sand like texture develops. There will be a few bigger crumbs, but most should be finer, smaller pieces. Place the bowl in the fridge until the coffee cake is ready for topping.
1 cup (120 g) all purpose flour, ½ cup (100 g) light brown sugar, ¼ cup (50 g) sugar, 1 teaspoon cinnamon , 6 - 8 tablespoons melted butter
In another bowl whisk the light brown sugar and cinnamon together until well combined. Set aside for now.
6 tablespoons (75 g) light brown sugar, 2 teaspoons cinnamon
In a medium bowl whisk the all purpose flour, baking powder, baking soda and salt together until well combined.
2 cups (240 g) all purpose flour, 2 teaspoons baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large mixing bowl, mash the bananas until mostly smooth with very few lumps. Whisk in the melted butter, light brown sugar, sugar, eggs, vanilla extract and buttermilk until well combined. Whisk in the dry ingredients until just combined.
1 cup mashed banana , ½ cup (113 g) melted butter, ½ cup (100 g) light brown sugar, ½ cup (100 g) granulated sugar, 2 large eggs, room temperature, 2 teaspoons (10 ml) vanilla extract , ½ cup (120 ml) buttermilk
Evenly spread half banana cake batter into the prepared pan. Top the banana cake layer with the cinnamon sugar. Repeat with a second layer of coffee cake. Top with the streusel.
Bake the coffee cake for 45 - 60 minutes or until a toothpick inserted into the center of the banna crumb cake comes out clean or with a few moist crumbs. Cool the pan in the pan for 15-20 minutes. Gently lift the coffee cake out of the pan using the parchment paper. Transfer to a serving plate or serve right from the pan. Make the vanilla glaze by whisking powdered sugar with milk or heavy cream. Top the banana cake coffee cake with the glaze, slice and enjoy!
1 cup (120 g) powdered sugar, 2 - 3 tablespoons (30 - 45 ml) milk or heavy cream