Preheat the oven to 350 degrees. Line an 8x8 square pan with parchment paper so that it goes up the sides. Set aside.
Make the blondies: In a large bowl, beat together whisked egg, cashew butter, mashed banana, coconut sugar, and vanilla with an electric mixer.
In a separate bowl, combine the dry ingredients: both flours, cinnamon, baking soda, sea salt. Stir the dry ingredients into wet and mix until fully combined and no dry pockets remain.
Pour the batter into the prepared pan.
Make the streusel: stir together the walnuts, almond flour, coconut sugar, and cinnamon. Pour the oil, butter, or ghee on top. Stir to combine. Spoon over the blondie batter.
Transfer to the oven and bake for 22-24 minutes, or until lightly golden and slightly firm. Remove from oven and let cool for at least 30 minutes before slicing into squares and serving.