Gluten-Free Brownies

Preheat the oven to 350°F (177°C).
Line a 13-by-9 inch (33-by-22 cm) baking pan with parchment paper or foil, leaving a couple inches overhang on all sides and butter the parchment or foil.
In a small bowl mix together the baking soda, salt, tapioca starch, and cassava flour (or tapioca starch or flour).
Place a medium heatproof bowl on top of a pot of simmering water, making sure the bottom of the bowl does not touch the simmering water. Add the chocolate and butter to the bowl and wait patiently while they melt, stirring often, until it's completely smooth. Carefully remove the bowl from atop the simmering pot of water and let the mixture cool slightly.
In a large bowl with a stand or hand mixer on medium speed, beat the eggs, vanilla, and sugar until the mixture thickens and begins to turn a pale shade of tan, 4 to 6 minutes.
Add the flour mixture, switch to a spoon, and stir just until incorporated. Stir in the melted chocolate and butter, mixing until everything is smooth and completely incorporated.
Scrape the batter into the prepared baking pan and bake for about 25 minutes, until the top looks dry, shiny, and cracked.
Let the brownies cool completely in the pan on a wire rack before cutting them into 2-inch (5-cm) squares. If desired, dust with cocoa powder or gluten-free confectioners' sugar.