Make the cake: Preheat the oven to 350 degrees F/180 C/gas 4. Lightly butter two 9-inch/23-cm round cake pans/tins. Line with parchment/baking paper. Butter the parchment/baking paper. Dust the inside of both pans with cocoa powder and knock against the side of the counter to settle a thin dusting of cocoa inside the pans; discard any extra cocoa.
Whisk together the flour, sugar, cocoa, baking soda/bicarbonate of soda, baking powder, and salt together in the bowl of a stand mixer.
Add 1 cup/240 ml of the warm water, the oil, and melted butter and mix on medium speed until combined. Stop the mixer to scrape down the sides of the bowl and make sure all the ingredients are fully incorporated. Add the eggs, one at a time, mixing well between additions to incorporate and aerate the batter, which gives the cake a more even crumb. Add the remaining 3/4 cup/180 ml warm water and vanilla and beat until smooth and aerated, about 2 minutes. The batter will look loose and watery, about the consistency of heavy cream or hot fudge.
Divide evenly between the prepared pans and tap lightly on the counter to smooth the top. Bake until the cakes are evenly domed and spring back lightly when touched, about 40 minutes. Test the cakes with a toothpick inserted into the centers; if it comes out clean your cakes are done.
Remove the cakes from the oven and let cool on a wire rack until they are just warm to the touch, about 15 minutes. The cakes will pull away from the sides of the pans slightly as they cool. Flip the pans to unmold the cakes onto a rack to cool completely, 1 hour.
Make the frosting: Before you begin, check to make sure the butter is room temperature, just soft enough to press easily with your finger. Beat together the butter, agave, confectioners'/icing sugar, cocoa powder, and warm water with a stand mixer until light and creamy and evenly combined, about 2 minutes. Stop to scrape down the sides and make sure all the butter is fully combined. Add the cool cream and beat on medium-high until fluffy and creamy.
When the cakes are completely cooled, slice off the dome from one of the layers with a serrated knife so that you can stack the layers easily. Lay the trimmed cake layer on a cake stand or serving platter, trimmed-side up. Tear four sheets of parchment/baking paper or wax/greaseproof paper into long strips and tuck under the edges of the cake around all the sides (this will help to keep your serving platter clean as you frost). Scoop about one-third of the frosting onto the top of the cake. Using an offset spatula or the back of a spoon, spread the frosting evenly over the top of the cake and around the sides. Cover with the second cake layer, domed-side up, and press down slightly to flatten. Look down at the cake and slide the top cake layer until it lines up perfectly with the bottom layer. Add another one-third of the frosting to the top and spread evenly over the top. Spread the remaining one-third of the icing around the sides of the cake to create a smooth, even layer.
Make careless swirls of icing all over the top of the cake using the back of a large spoon. Serve immediately or store in a cool place until ready to serve.
If you plan to make ahead and refrigerate, note that the icing will set up in the fridge and will sweat slightly when it comes out. Remove from the fridge about 1 hour before serving to restore the frosting to its luscious, creamy state.
P.S. If you want a deep, dark chocolate flavor, use Dutch-process cocoa. For an old-fashioned-quality cake, use natural cocoa.
Reprinted with permission from The Newlywed Cookbook by Sarah Copland, Chronicle Books copyright (c) 2012
Photograph by Sara Remington