In a large bowl, beat together the butter, oil, sugar, baking powder, baking soda, and salt until smooth.
Beat in the eggs.
Beat in the oat flour, then the buttermilk, then finish by beating in the almond flour. Mix in the cocoa powder until smooth.
Fold in the zucchini and spoon the batter into the prepared pan. Rest the batter for about 20 minutes.
Preheat the oven to 325°F. Bake the cake for about 50-60 minutes, until the top springs back lightly when touched and a toothpick inserted in the center comes out clean.
Remove the cake from the oven and cool in the pan on a wire rack.
To make the icing: In the microwave or on the stovetop, combine the chocolate and cream until the chocolate softens, then stir to mix in the the chocolate. I did about 1 minute in the microwave. Spread the icing over the cooled cake.
Oat flour substitute - Try brown rice flour, teff flour, or sorghum flour by weight.
Almond flour substitute - You can use another nut flour, brown rice flour, or tiger nut flour.
Make it dairy-free - Use dairy-free yogurt for the buttermilk or add 1 teaspoon of vinegar to a non-dairy milk of choice. For the icing, use coconut cream instead of the cream.
Other icing choices - You can make the ganache as written (recommended by me!) or swap it with a simple chocolate glaze. You can also sift powdered sugar over the top.
Rest the batter - As always, gluten free cake batters very much benefit from a 20-30 minute rest after they've been assembled but before baking. This allows the gluten free flours to fully hydrate, making the finished cake taste so much better!
Refined sugar-free - Substitute coconut sugar for the sugar in the cake.
Egg-free - I haven't tested this recipe with an egg substitute, but you could try using flax eggs instead of the eggs.
Make a layer cake - Bake the cake in two 9-inch layer cake pans (note that baking time may differ), then fill and frost the cake with your favorite chocolate frosting (may I suggest my recipe for chocolate buttercream?)