Sift together flour, sugar, Truvia, baking soda, xanthan gum and cocoa.
Add vinegar, (it will foam when it comes in contact with the baking soda). Combine vanilla extract with water and oil and add to dry ingredients.
Mix with an electric mixer on the lowest speed just until combined well or mix by hand. Bake in an 8x8-inch pan (preferably glass) lightly sprayed with non-stick spray at 350°F for 25 minutes, or until a toothpick inserted into the middle comes out clean.
This cake tends to taste sweeter when it cools off. Tastes great frosted with peanut butter!