Preheat oven to 350 degrees F.
In a bowl, whisk together teff flour, walnut meal, sweet rice flour, salt, pumpkin pie spice, baking soda, baking powder, and sugar. Set aside. In another bowl, whisk together sweet potato puree, egg, olive oil, and maple syrup.
Pour into a parchment lined 6-inch round springform pan. Smooth out.
In a small bowl, mix together all crumble ingredients, using your hands to disperse butter throughout the mixture, until you have a thick crumble. Sprinkle on top of cake evenly.
Make a small well in the middle of the dry mixture, and pour in wet ingredients. Gently stir together, until no streaks of flour remain.
Bake for 40 minutes at 350 degrees F. Remove and let cool in pan for 10 minutes, then release the sides and let finish cooling. Dust with powdered sugar when ready to serve.