Preheat the oven to 350°F (175°C) and line or grease an 8x8-inch baking pan.
Mash bananas in a mixing bowl. Add the egg, vanilla, and stevia. Mix until smooth.
Add baking soda, baking powder, salt, and cinnamon. Stir to combine.
Fold in almond flour and crushed walnuts until a thick batter forms. Pour into the prepared pan.
Make the streusel: In a separate bowl, mix crushed walnuts, coconut oil, maple syrup, almond flour, and cinnamon until crumbly.
Sprinkle the streusel evenly over the cake batter.
Bake for 25 minutes, or until a toothpick comes out clean and the top is golden.
Let cool slightly before serving. Optional: Drizzle with a dairy-free vanilla glaze if you like it extra sweet!