Preheat the oven to 350 degrees F (177 degrees C).
Grease a 8x8 baking pan with butter and line with parchment paper. Set aside.
Heat the butter over high heat until it melts. Lower the heat to low. Heat the butter until small brown bits start to appear. Remove from the stove.
Add dark chocolate to a small bowl. Pour the brown butter over the top and stir until all the chocolate melts. Let the mixture cool for 2 minutes.
In large bowl, combine granulated sugar, brown sugar, eggs, vanilla extract, and salt. Beat with a hand mixer or stand mixer until thick and creamy (~5 minutes).
While mixing, stream in your chocolate ganache from step 4. Mix until well combined.
Lastly, add your sweet rice flour and cocoa powder to the bowl. Use a spatula to combine.
Spoon batter into your prepared baking pan. Smooth it out with your spatula.
Dollop spoonfuls of the tahini on top of the brownie batter. Use a knife to swirl the tahini into the chocolate. Move the knife in small circles around the brownie batter to achieve the prettiest swirls.
Bake for 20 minutes.
Take out the brownies and smack them on the counter to get rid of some the air bubbles. Sprinkle some flaky sea salt over the top and put the brownies back in the oven. Bake for another 10 minutes.
Let cool for 10 minutes before serving.