Step 1

To make the streusel, in small bowl, whisk together the flour, sugar, cinnamon and salt. Scatter the butter over the top and, using your fingers, rub it into the dry ingredients until the mixture resembles coarse crumbs. Freeze, uncovered, until ready.

Step 2

To make the cake, heat the oven to 350°F with a rack in the middle position. Mist an 8-inch square metal baking pan or glass baking dish with cooking spray, then line it with a 7-by-14-inch piece of kitchen parchment with the excess hanging over the edges.

Step 3

In a small bowl, whisk together the flour, baking powder and salt. In a medium bowl, whisk the butter and sugar. Whisk in the eggs, milk and vanilla. While whisking, gradually add the dry ingredients and mix until just combined. Pour half of the batter into the prepared pan and spread evenly. Place the banana slices on top, overlapping as needed. Top with the remaining batter, spreading it evenly. Sprinkle the streusel over the batter, evenly covering the surface.

Step 4

Bake until the streusel is golden brown, 45 to 50 minutes if using a metal pan or 55 to 60 minutes if using a glass dish. Cool completely in the pan on a wire rack, about 1½ hours. Using the parchment overhang, lift the cake out of the pan and set on a cutting board. Cut into 12 to 16 squares. If desired, dust with powdered sugar just before serving.