Preheat oven to 350 degrees F. Line the inside of 9×5 inch baking pan non-stick foil.

Stir together the rice flour, tapioca flour and salt; set aside.

In a saucepan, melt butter over medium heat. When butter starts to melt and coats pan bottom, add unsweetened chocolate and reduce heat to low. Stir constantly until chocolate is melted. Add ½ cup of the sugar to melted chocolate and stir over low heat for about 40 seconds. Remove chocolate mixture from heat and let cool for about 5 minutes. Stir in vanilla.

Meanwhile, using an electric mixer (hand-held or stand okay), beat eggs for a full 2 minute or until foamy and light. Gradually add remaining ½ cup of sugar to eggs and continue beating for another minute.

Using a rubber spatula, fold a small amount (about ⅓ cup) of the egg mixture into the warm chocolate mixture, then fold the lightened chocolate into the bowl with the egg mixture. Stir in the rice/tapioca flour/salt mixture, being careful not to over-mix. Pour into pan and bake for 25-28 minutes or until top is set. Let cool completely. Transfer to refrigerator and chill for a few hours or overnight. Lift from pan and cut.