Preheat oven to 350°F. Grease and line three 8" cake pans. Whisk together flour, baking powder, baking soda and xanthan gum. In a small bowl, mix flax seed and water. Set aside 5min. In a small bowl, mix coconut milk and cider vinegar. Set aside 5min. In a large bowl, beat sugar and shortening until light and fluffy. Add flax seed mixture and vanilla. Add half the flour mixture, all the vinegar mixture, and the remaining flour mixture, beating with each addition. Bake 25 min. until toothpick comes out clean. Frost with icing of choice.