1/4cup (30 grams)cassava flour (or substitute tapioca starch or flour)
1pound (454 grams)chocolate, chopped (semisweet or dark up to 85%, depending on desired sweetness)
6tablespoons (3 ounces or 85 grams)unsalted butter, plus more for the pan
2teaspoons (8 ml)vanilla extract
1cup (200 grams)coconut sugar
Cocoa powder or confectioners sugar, for dusting (optional)
1. Preheat the oven to 350°F (177°C).
2. Line a 13-by-9 inch (33-by-22 cm) baking pan with parchment paper, leaving a couple inches paper overhang on all sides.
3. In a small bowl mix together the baking soda, salt, tapioca starch, and cassava flour (or tapioca starch or flour).
4. 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 is completely smooth. Carefully remove the bowl from atop the simmering pot of water and let the mixture cool slightly.
5. In a large bowl with a stand or hand mixer on medium speed, beat the eggs, vanilla, and sugar until the mixture begins to turn to a pale tan and thickens, 4 to 6 minutes. Add the flour mixture, switch to a spoon, and beat just until incorporated. Pour the melted chocolate and butter into the mixture while stirring. Mix until everything is completely incorporated and smooth.
6. Scrape the batter into your prepared baking pan and bake for about 25 minutes, until the top looks dry, shiny, and cracked. Let it cool completely before cutting into 2-inch (5-cm) squares. If desired, dust with cocoa powder or gluten-free confectioners’ sugar.