Preheat oven to 350°F and line a metal loaf pan with parchment paper. If using a glass pan, preheat oven to 325°F.Tip: You can double the recipe and make it in a square pan.
Make the streusel: In a small bowl, combine oat flour, brown sugar, and spices. Add walnut butter and soy milk/pumpkin puree and mix. Use your fingers to pinch the mixture together to form crumbles. Set aside while you prepare the cake.
Whisk wet ingredients: In a large mixing bowl, whisk together pumpkin puree, soy milk, walnut butter, brown sugar, vinegar, and vanilla extract.
Add spices: Whisk in pumpkin pie spice.
Add dry ingredients: Add oat flour, buckwheat flour, and baking soda and mix until well combined.
Bake: Add the batter to your lined baking pan and smooth out the top. Sprinkle streusel on top. Bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean and the top of the cake feels firm. Check the cake after 30 minutes and tent with foil if the streusel is browning too quickly.
Cool: Place the pan on a wire rack to cool for 20 minutes, then carefully lift the cake out of the pan (with the parchment paper) and place on the wire rack. Gently slide the cake off of the parchment paper and let the cake cool on the wire rack. Let the cake cool completely before you cut it or the structure of the cake may not set.
Serve: The cake itself is very lightly sweetened, so if you want it sweeter you can add the glaze. Place powdered sugar in a bowl and add milk ½ teaspoon at a time while whisking, until thinned to your liking. Drizzle the glaze on once the cake has cooled.
Store: Store in an airtight container or wrapped tightly in plastic wrap for up to 2 days at room temperature or in the fridge for 5 days. If you'd like to freeze it, leave the cake unglazed, cut into slices, and freeze in freezer safe containers for about 2 months.
Notes
1) Pumpkin Pie Spice can be replaced with ½ tsp cinnamon + ⅛ tsp nutmeg + ⅛ tsp ginger + ⅛ tsp all spice + ⅛ tsp cloves (for 1 teaspoon of pumpkin pie spice; cut amounts in half for streusel). 2) See how to make your own walnut butter here.