Pre-heat oven to 350°. Line an 8×8″ baking dish with parchment paper (easiest if you use 2 pieces of parchment that overlap and leave a 3” overhang on the sides)
Whisk tapioca flour and cocoa powder in a medium bowl until no lumps remain.
Heat chocolate and coconut oil 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 eggs, granulated monk fruit, and coconut sugar in a large bowl until light + smooth, about 3–4 minutes.
Beat in salt and vanilla 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 parchment lined baking pan + bake until a cake tester or toothpick comes out clean, approximately 20-25 minutes.