Preheat Oven, Make Buttermilk: Preheat your oven to 350 degrees Fahrenheit. Line an 8-inch round cake pan with parchment paper and set aside. Whisk together the oat milk and lemon juice.
Combine Dry Ingredients: In a large bowl, combine the brown sugar, coconut sugar, salt, baking powder, baking soda, cocoa powder, flour, and orange zest.
Combine Wet Ingredients, Mix With Dry: Add the orange juice, oil, and eggs to the oat milk mixture and whisk. Pour the wet ingredients into the dry ingredients and mix well.