Preheat oven to 350 degrees F. Grease an 8 inch metal pan and line with foil. Grease bottom oil with extra butter or spray with cooking spray.
Begin melting the butter in a small saucepan over medium heat. When it is about halfway melted, reduce heat to low and add the chocolate chips. Melt the chips, then stir in the cocoa powder and 1/3 cup of the sugar (the brown, if using two kinds). Remove from heat and stir until smooth.
In a mixing bowl, using a handheld electric mixer, beat together the eggs, the other 1/3 cup of sugar, salt and vanilla.
Stir the melted chocolate mixture into the egg mixture, then stir in the tapioca flour.
Spread batter in pan and bake for 25-27 minutes or until top appears set.
Let cool at room temperature for several hours or let cool slightly at room temperature and finish cooling in the refrigerator.
Lift from pan and cut into squares.