Preheat oven to 350°F (175°C) and grease/flour two 8-inch round cake pans. In a medium bowl, whisk together the unsweetened soy milk and apple cider vinegar; set aside for 5 minutes to curdle.

In a large bowl, combine the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt. Whisk well to combine and break up any lumps.

To the curdled soy milk, add the canola oil, unsweetened applesauce, and 1 tablespoon of pure vanilla extract. Whisk thoroughly until well combined. Pour the wet mixture into the dry ingredients and mix with a spatula until just combined.

Carefully stir in the 1 cup of boiling water until the batter is smooth. Do not overmix. Divide the batter evenly between the prepared cake pans.

Bake for 30-35 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.

In a large bowl using an electric mixer, cream together the softened vegan butter until light and fluffy. Gradually beat in 1 cup of cocoa powder and 4-5 cups of powdered sugar, alternating until well combined.

Add 2 teaspoons of pure vanilla extract and 1/4-1/2 cup of unsweetened soy milk, starting with less and adding more as needed. Beat on high speed until the frosting is smooth, creamy, and reaches your desired consistency.

Once the cake layers are completely cooled, frost the cake with the prepared chocolate buttercream. Stack the layers and apply an even layer of frosting to the top and sides of the cake. Decorate as desired.