Make the streusel: In a small bowl, combine oat flour, brown sugar, and cinnamon. Break up any clumps of brown sugar. Add walnut butter and soy milk and stir until the mixture starts to clump together. Set aside.
Add the dry ingredients to the wet and mix together just until no dry flour is left. Cover and let the batter rest for 20-30 minutes.
Preheat oven to 350°F. Line an 8x8-inch pan with parchment paper.
Transfer half the cake batter to the lined pan. Sprinkle cinnamon filling on top. Cover with remaining cake batter. Crumble the streusel on top and gently press the streusel into the cake batter to help it stick.
Bake for 35-45 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Place the pan on a wire rack to cool for 15 minutes, then use the ends of the parchment paper to carefully remove the coffee cake from the pan. Place directly on the wire rack to finish cooling.Note: The streusel will be very crumbly at first but will stick more as the cake cools.