In a large glass bowl, melt the vegan butter/coconut oil. Add the flax/water mixture, orange zest sugar, salt, vanilla, and cocoa powder and whisk until fully combined. Pour in the room temp/warmed orange juice and mix again. Add the gluten-free flour, baking powder, baking soda, and oat flour and whisk very well. Scrape the sides and bottom of the bowl with a spatula. Pour in the warm water and give the batter one last good mix. Transfer the batter to the prepared pan, and bake for 25-30 minutes or until a toothpick comes out clean. Remove from the oven and let it cool mostly at room temperature.