Grease a 9-inch by 9-inch brownie pan, and heat the oven to 350 F.
Dump the tigernut flour, cocoa, arrowroot starch, vanilla bean powder and sea salt into a small bowl and whisk them together until thoroughly combined. Let the bowl alone while you prepare the other ingredients.
Fill a saucepan with water and bring it to a slow boil. Set a tempered glass or pyrex bowl over the simmering water to create a double boiler. Melt the butter and sugar together in the top bowl. When the butter melts, whisk it into the sugar until it forms a smooth and uniform paste, then stir in the chocolate. Continue stirring the chocolate into the butter and sugar until it melts.
Immediately remove the bowl from the heat and beat in the eggs. The batter will stiffen slightly when the eggs are fully incorporated, then beat in the dry ingredients until they're thoroughly and uniformly combined. Pour the batter into a prepared brownie pan and bake for 20 minutes, cool completely, and then slice and serve.