Preheat the oven to 350°F. and line an 8-inch square baking dish with parchment paper.
Make the streusel topping first. Combine all streusel ingredients in a small bowl and set aside.
In a medium bowl, mash the bananas until no lumps remain, then whisk in the sweetener, melted butter or oil, eggs and vanilla extract.
In a large bowl, whisk the almond flour, tapioca starch, baking powder and salt.
Add the wet ingredients to the dry ingredients, using a spatula to combine until no traces of flour remain.
Transfer the batter to the lined baking pan, smoothing to fill the corners. Sprinkle the streusel topping evenly over the top, pressing it down gently.
Bake for 35-40 minutes or until a toothpick inserted in the center of the cake comes out clean, or almost clean.
Let the cake cool in the pan for 30 to 60 minutes, then carefully lift it by the edges of the parchment paper to a cooling rack. Slide the parchment paper out from beneath the cake and let it cool completely before cutting transferring it to a serving plate.