In a large bowl combine flours, sugar, baking powder, xanthan gum, and cacao powder.
Next, add soy milk, oil and combine. The batter should be creamy.
Pour the batter into 8 inches square baking pan lined with parchment paper. Bake at 400F (200°C) For about 35-40 minutes.
Meanwhile, let's make the vanilla frosting.
In a medium-sized saucepan cook rice flour with milk and 2 tbsp of brown sugar. Bring it to a boil and cook until the mixture is thick.
Let this mixture cool completely before moving on to the next step.
In a large bowl whisk margarine with powdered sugar until soft and airy. Slowly add the rice flour mixture and vanilla, and whisk until everything is combined.
When the cake is baked and cooled down to room temperature we can now decorate the cake. Just spread the vanilla frosting on top and put the cake in the fridge for at least 1 hour.
Now it's time for chocolate decorations. Simply melt the dark chocolate and use a small spoon to create bones, ghosts, and pumpkins. The shapes are quite easy and simple. Draw the shapes onto parchment paper so they are easy to peel off. When the shapes are done, put them in a fridge for about 20 minutes.
Serve the cake with spooky chocolate decorations and enjoy!