Preheat the oven to 375°F. Lightly grease an 8-inch square baking pan with canola oil.

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

In a large bowl, mix together the baking mix, 1 cup of the sugar, the baking powder, baking soda, and xanthan gum. Using a pastry blender or 2 knives, blend in the shortening until the mixture resembles coarse meal. Then blend in the flaxseed mixture, the “buttermilk” mixture, and the vanilla.

Pour the batter into the prepared pan and smooth it out. Mix the remaining 1 tablespoon of sugar and the cinnamon in a small bowl, then sprinkle the cinnamon sugar over the top of the batter.

Bake the cake for 20 to 25 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan on a cooling rack before slicing it and serving.

Store the cooled cake, tightly wrapped in an airtight container and refrigerated, for up to 3 days.