Position a rack in the center of the oven and preheat the oven to 350°F. Lightly grease three 8-inch round cake pans with canola oil and line with parchment paper.

In a small bowl, combine the water and flaxseed meal and allow to thicken for 3 to 5 minutes. In another small bowl, make “buttermilk” by combining the coconut milk and cider vinegar.

In a large mixing bowl, mix together the baking mix, baking powder, baking soda, and xanthan gum. In the bowl of a stand mixer, beat the sugar and shortening until they are light and fluffy. Then blend in the flaxseed mixture and the vanilla. Alternately stir in the dry ingredients and the “buttermilk” mixture, starting and ending with the dry ingredients.

Pour the batter into the prepared pans and smooth it out with a knife or spatula. Bake the cakes for 25 minutes, or until the tops are golden and a toothpick inserted in the centers comes out clean.

Let the cakes cool completely in the pans on a cooling rack. Turn the cake layers out and frost the tops of each layer with the frosting. Transfer 1 layer to a serving plate and stack the other layers on top. Frost the sides of the cake.

Store the frosted cake, covered and refrigerated, for up to 3 days.