Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch springform pan and line with parchment paper.
Combine teff flour, rice flour, arrowroot, sugar, coffee, baking soda, and salt in a large bowl.
Place chocolate and 2 tablespoons soy milk in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Add 2 cups soy milk and stir until well combined. Add vinegar.
Pour the chocolate mixture over the flour mixture and mix with a spatula until batter is well combined. Pour into the springform pan and level with a spatula.
Bake in the preheated oven until set but still slightly soft, about 35 minutes. Allow to cool for a few minutes in the pan, then remove and cool completely on a wire rack, about 45 minutes.
Make the glaze while the cake is cooling. Place chocolate and coconut oil in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Pour melted chocolate over cake and spread evenly. Allow to set before slicing.