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.
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.
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.
Bake for 40 minutes at 350 degrees F. Test with toothpick and if no wet batter sticks on toothpick, remove and let cool in pan for 10 minutes, then release the sides and let finish cooling. If you do get wet batter on the toothpick, bake in increments of 3 minutes, until just moist crumbs stick on the toothpick. Dust with powdered sugar when ready to serve.