Set the oven to 350°F (175°C). Lightly grease a 9×13-inch pan, and then line the bottoms with parchment paper so the cake doesn’t stick. Set them aside.

In a medium bowl, stir together the coconut flour, almond flour, xanthan gum, baking soda, baking powder, and salt. Break up any lumps as you go. Once everything’s combined, set the bowl aside.

In a larger bowl, whisk the mayonnaise, eggs, vanilla, and Gentle Sweet until the mixture is smooth. Slowly pour in the water while stirring until everything blends together evenly.

Add the dry ingredients to the wet mixture a little at a time. Stir by hand or use a mixer on low just until the batter is smooth. Let it sit for a few minutes so the coconut flour can soak up the moisture and thicken slightly.

Pour the batter into your prepared pan(s), spreading it out evenly. Bake for 30 to 35 minutes, or until a toothpick poked into the center comes out clean. Let the cake cool in the pan for 10 minutes, and then move it to a wire rack to finish cooling before adding any frosting or toppings.