Preheat the oven to 350 degrees F (180 degrees C). Line an 8×8″ baking dish with parchment paper.
Whisk Tapioca Flour (5 tablespoon) and Unsweetened Cocoa Powder (5 tablespoon) in a medium bowl until no lumps remain.
Heat Dark Chocolate (12 ounce) and Coconut Oil (6 tablespoon) in a small saucepan over low, stirring, until melted and smooth. DO NOT bring to a boil!
Using an electric mixer on medium-high speed, beat Large Egg (4), Granulated Monk Fruit Sugar (2/3 cup), and Coconut Sugar (1/2 cup) in a large bowl until light and smooth, about 3–4 minutes.
Beat in Kosher Salt (1 teaspoon) and Vanilla Extract (2 teaspoon) to combine, then beat in chocolate mixture, scraping down sides as needed, until incorporated.
Beat in tapioca flour mixture, then increase speed to medium-high and beat until mixture is thick and holds its shape, about 30 seconds.
Pour into a parchment-lined baking pan and bake until a cake tester or toothpick comes out clean, approximately 20-25 minutes.
Once ready, let cool and serve.