Preheat the oven to 350°F (175°C). Grease and flour an 8-inch (20 cm) round cake pan.
In a large bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt. Set aside.
In a separate medium bowl, whisk together the vegetable oil, unsweetened applesauce, erythritol sweetener, eggs, and vanilla extract until smooth and well combined.
In a small bowl, combine the unsweetened almond milk and apple cider vinegar. Let it sit for 5 minutes to create a vegan buttermilk substitute.
Add the almond milk mixture to the wet ingredients and mix well.
Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Do not overmix.
Carefully stir in the hot water until the batter is smooth. The batter will be thin; this is normal.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 10 minutes.
Transfer the cake to a wire rack to cool completely before frosting or serving.