Begin by preheating your oven to 350°F (175°C). Line a 9x13 inch (23x33 cm) baking dish with aluminum foil and lightly spray it with cooking spray.
In a medium bowl, combine the flour, baking powder, and salt.
Using a stand mixer with a paddle attachment, mash the bananas until smooth. Blend in the softened butter, then add the sugar, eggs, milk, and vanilla. Gradually mix in the flour until just combined.
To make the streusel, mix all the ingredients in a medium bowl, cutting in the cold butter until a coarse crumb forms.
Pour half of the batter into the prepared pan, sprinkle with a third of the streusel, then add the remaining batter and top with the rest of the streusel. Bake for 50-55 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool for at least 1 hour.
For the glaze, whisk together the powdered sugar, vanilla, and milk, then drizzle over the cooled cake before serving.