Position a rack in the center of the oven and preheat the oven to 350°F. Lightly spray a 9x13-inch cake pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on the pan's two long sides. Spray the parchment, too.

Place the butter and cocoa powder in the top of a double boiler or in a heatproof bowl set over a medium, heavy-bottomed saucepan filled with a few inches of simmering water (be sure the bottom of the bowl does not touch the water). Cook over medium heat to melt the butter. While the butter melts, in a food processor, pulse the chocolate for a. few seconds at a time until roughly chopped. Add the chocolate to the butter and cook, using a. heatproof silicone spatula to stir the mixture and scrape the sides of the bowl occasionally, until the butter and chocolate halve melted and combined, about 10 minutes. Set the double boiler or bowl on a wire rack and let the chocolate mixture cool while you prep the other ingredients.

In a small bowl, whisk together the buckwheat flour, sweet rice flour, tapioca starch, and salt.

In the bowl of a stand mixer fitted with the whisk attachment, combine the eggs, sugars, and vanilla. Whisk on low until combined, 1-2 minutes, using a silicone spatula to scrape down the bottom and sides of the bowl as necessary. Increase the mixer to medium and slowly pour in the chocolate mixture, aiming for the sides of the bowl as opposed to the whisk. Continue whisking on medium until the batter is smooth, about 1 minute.

Sprinkle the dry ingredients over the batter all at once and use a silicone spatula to mix until just combined.

Pour the batter into the prepared pan and smooth the top. Bake for 35-40 minutes, or until a. skewer inserted into the center comes out with just a few crumbs attached. Cool completely on a wire rack before slicing. It's better to pull the brownies out of the oven early than leave them in too long - if you over bake the brownies they'll be tough. They might appear underbaked, but they'll be perfect when cooled.

Run a butter knife or offset spatula along the edges of the pan and use the overhanging parchment as handles to lift the brownies out of the pan and onto a cutting board. Slice into 20 rectangles, each about 2¼ x 2½ inches, and serve. The brownies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.