Preheat oven to 180 degrees C. Grease a 23x33 cm tin and set aside.
In a medium bowl, sift together the sorghum, tapioca and rice flours with the cocoa powder, xanthan gum, baking powder and bicarb soda.
In a large mixer bowl, cream the butter until light and fluffy. Slowly beat in the brown and white sugars; whip until fluffy. Beat in the eggs and egg yolks one at a time. Add the vanilla. On low speed, alternately combine the buttermilk with the flour mixture. Pour batter into prepared tin.
Bake in preheated oven for 30 to 35 minutes or until a toothpick inserted into the centre of the cake comes out clean. Cool in tin.
To make the icing, in the top of a double boiler over medium high heat, melt the chocolate chips (or use microwave). Remove from heat and cool until warm. Stir in the sour cream and vanilla; add thickened cream. Stir in additional thickened cream to make desired consistency. Once the cake is thoroughly cool, spread a thin layer of icing over the top.