1Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch round cake pan; line with parchment paper.
2Whisk coconut sugar, oat flour, spelt flour, cocoa powder, baking soda, and salt together in a bowl. Pour in buttermilk, coconut oil, and vanilla extract; whisk until batter is well blended.
3Pour batter into the prepared pan; smooth top with a rubber spatula.
4Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 25 minutes. Cool in the pan for 5 minutes. Invert onto a wire rack to cool completely, about 25 minutes.