I got this recipe from “Gluten-Free Baking” by Simon & Alison Holst
• 3 large eggs
• 1/4 teaspoon cream of tartar
• 1/2 cup oil
• 1 cup plain unsweetened yoghurt
• 2 teaspoons vanilla essence
• 1 1/2 cups sugar
• 1 cup rice flour
• 1 cup tapioca four
• 1/2 cup cocoa powder
• 1 teaspoon guar or xanthan gum
• 1 teaspoon baking soda
• 125-150g dark chocolate
• 1/4 cup sour cream
• Preheat oven to 180°C.
• Separate the eggs and place the egg whites in a large bowl. Add the cream of tartar then beat the egg whites until they form stiff peaks.
• Place the yolks in another bowl or jug, then add the oil, yoghurt and vanilla essence then whisk to combine.
• Sift the dry ingredients into the egg whites, pour the yoghurt-yolk mixture over the dry ingredients, then gently fold everything together until just uniformly mixed.
• Pour the batter into a greased tin and level the top if required.
• Bake the cake for 40-50 minutes until a skewer comes out clean. Remove from the oven and allow to cool for 5-10 minutes before removing from the tin and cooling completely on a rack.
• To make the icing, gently heat the dark chocolate with sour cream until just melted, stir until smooth and glossy, then leave to cool until thick enough to spread.
• Leave the cake to stand until the icing is firm, then serve as is or with yoghurt or lightly whipped cream.
I got the method for the chocolate roses from this blog. It includes the recipe for chocolate plastic as well as a step-by-step photo method for making the roses.