Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan and set aside.
In a medium bowl, sift together the sorghum, tapioca, and rice flours with the cocoa powder, xanthan gum, baking powder, and baking 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 pan.
Bake in preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Cool in pan.
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 heavy cream. Stir in additional heavy cream to make desired consistency. Once the cake is thoroughly cool, spread a thin layer of frosting over the top.