Preheat the oven to 350°F. Liberally spray a 12-cup Bundt pan with cooking spray.
In a small bowl, prepare the flax eggs by whisking together the ground flax and water. Set aside for at least 10 minutes.
In a medium bowl, mix the coconut milk or almond milk and vinegar. Whisk and set aside.
In a large bowl or stand mixer, mix the almond flour, brown rice flour, buckwheat flour, coconut sugar, baking powder, baking soda, and sea salt. Stir to blend.
To the bowl with the milk and vinegar mixture, add the flax eggs, applesauce, coconut nectar or maple syrup, and vanilla extract. Whisk until creamy.
Pour the wet ingredients into the bowl with the flour mixture. While beating on medium speed, incorporate the melted coconut oil, continuing to beat until a batter is formed, increasing the speed as needed.
Transfer the batter to the Bundt pan. Bake for40 to 45 minutes until edges are golden brown, a toothpick inserted in the center comes out clean, and the internal temperature reaches between 200° and 210°F. Allow the cake to completely cool in the pan for about 2 hours.