Grease a 9-inch round baking tin and line it with parchment paper rounds.
Melt the chocolate in a bowl set over simmering water then set aside to cool.
In a large bowl, cream the butter using an electric mixer. Beat in the sugar until light and fluffy. Add the eggs one at a time, beating well to combine. Beat in the vanilla. Add the chocolate and mix to incorporate.
In a medium bowl, whisk together the teff flour, almond flour, baking soda, and salt. Add all at once to the chocolate mixture and mix to combine. Pour in the ice water and beat until a smooth batter forms.
Pour the batter into the pan, smooth the top, and preheat the oven to 350℉. Rest the batter while the oven heats, then place the cake in the oven and bake for 35-40 minutes, until a tester inserted in the middle of the cake comes out clean and the edges are starting to come away from the sides of the pan.
Remove cake from the oven and cool in the pan on a wire rack for 20 minutes, then turn out onto the rack to cool completely.
Make Frosting
Melt the chocolate and set aside to cool slightly. In a large bowl, whip together the heavy cream and chocolate until a smooth whipped frosting forms. Use it to frost the cake.
Notes
Tips
Take a rest - Make sure to rest the batter for 20 minutes, or while the oven heats to help the gluten free flour hydrate. This is such an important step in gluten free baking so try not to rush it!
Flour substitutions - As mentioned, oat flour is a great choice to substitute for teff flour. For the almond flour, you can try to get tiger nut flour or sorghum flour. Or, keep it even easier and substitute a gluten free flour blend for the total of both of the flours.
Make it dairy-free - Substitute your favorite non-dairy butter or melted coconut oil for the butter. Use coconut cream (not coconut milk) in the frosting instead of heavy cream.
Go refined sugar-free - Substitute coconut sugar for the sugar.
Another frosting - The rich cap of chocolate ganache is the perfect finish to this teff chocolate cake. But, feel free to swap any other frostings, like vanilla or chocolate buttercream, or top it with whipped cream or a simple dusting of powdered sugar.