Combine flour, sugar, cocoa, baking soda, baking powder, and salt in a large bowl.
Next, melt coconut oil and set aside, allowing it to cool for a few minutes.
In a separate bowl, whisk milk, applesauce, vinegar, and vanilla together; add coconut oil.
Combine wet and dry ingredients with an electric mixer.
Next, line three 6" cake pans with coconut oil and then distribute cake batter evenly into pans.
Bake cakes for approximately 35-40 minutes, or until a toothpick comes out clean.
Take cakes out of oven and wait 10 minutes before carefully transferring them onto cooling racks, allowing them to cool completely before icing them.
For the icing: Follow the instructions in this post to make the homemade Nutella. Then, add 3/4 cup homemade Nutella to a bowl and stir in the butter and powdered sugar until smooth.
To assemble: Use a serrated knife to remove "domes" from each cake, making all three cakes flat on the top. Next, place a good amount of icing on the one cake and smooth it out, and then place the second cake on top of the icing layer, and repeat that process for the final layer.
Then, place icing all along the sides until the whole cake is coated with icing.