Preheat the oven to 350°F. Grease two 9-inch round cake pans with canola oil, line the bottoms of the pans with parchment paper, and then lightly grease the parchment paper.
In a small bowl, make “buttermilk” by combining the rice milk and cider vinegar. In a large mixing bowl, combine the baking mix, salt, and xanthan gum.
In the bowl of a stand mixer, cream together the sugar and shortening. Meanwhile, in a separate small bowl, mix the cocoa powder and red food coloring to form a paste; set the paste aside. Add the applesauce and vanilla to the creamed shortening and sugar and blend until the mixture is light and creamy. Add the cocoa paste and mix well.
Alternately add the dry ingredients and the “buttermilk” mixture to the batter, beginning and ending with the dry ingredients.
Pour the white vinegar into a small cup and sprinkle the baking soda over it, Immediately pour the fizzing combination into the cake batter and thoroughly combine the two mixtures.
Divide the batter between the prepared pans and bake the cakes for 30 to 35 minutes, or until a toothpick inserted in the centers comes out clean.
Let the cakes cool completely in their pans on a cooling rack, Turn the cakes out of their pans and remove the parchment paper. Frost the top of each cake layer with the frosting, Transfer 1 layer to a serving plate, and then set the other layer on top, Frost the sides of the cake.
Store the frosted cake, covered and refrigerated, for up to 3 days.