First, make the cake layers. Prepare the cake pans: oil them well, dust lightly with flour, then cover the bottom of each with a circle of parchment paper and oil that. (If you are using a different size of pan, prepare it similarly; if you are using cupcake tins, just line with paper cups. See Kitchen Notes for different sizes of pans.)
Heat the oven to 350ºF. Position the racks in the center.
Spoon the flour into a large mixing bowl, then lightly add in the cocoas powder, baking soda, baking powder, salt and cayenne pepper, if using. Stir everything with a fork.
In a medium bowl, crack the eggs, whisk them quickly, then add in the buttermilk and vanilla and then the melted plant butter, mixing at each step. Add all of this to the dry ingredients. Mix everything at medium speed with your electric hand mixer—90 seconds to 2 minutes—you want everything to be uniform and non-lumpy. Then pour in the boiling water and mix it into the batter with the mixer, at low speed, or just with a whisk. The batter will be very thin.
Divide the batter between the two prepared cake pans—I used a half-cup measure to dole things out evenly. Put the pans in the oven—one per rack. In about 15 minutes, gently open the oven door and switch each to the other rack, also rotating them a half turn. In another 10 (or so) minutes, test them—a tester inserted at the center should come out clean and the cake should have very slightly pulled away from the sides of the pan. If they are not ready, close the oven door gently and wait another minute or two.
When the layers are baked, let them cool on a rack for about 15 minutes. Then carefully remove them from the pans and cool them completely on the rack. Don’t frost until everything is completely cooled.
Leveling the cake: if your cake layers came out very lopsided, or puffy in the middle, then at this point, I recommend leveling the layers for the sake of symmetry. There are bunches of videos on youtube about how to do this; I like this one which is quite simple. Make the frosting when you intend to ice the cake—this is not a thing you can make the day before. To make the frosting, mix together the cocoa powder and the melted plant butter in a medium bowl, using a whisk. Then add in the powdered sugar, 1/3 at a time, the vanilla, the cayenne and cinnamon (if you are using them) and the milk. All of this will quickly change from a bunch of different things into a nice smooth frosting.
For a two-layer cake, set the bottom layer on whatever you intend to use as a serving dish. I used the cake stand shown in the photo. I also used the chocolate icing for the center filling. You could also use a jam—next time I will likely use a thin layer of raspberry or cherry jam. Once you’ve created a nice, uniform coat, carefully add on and center the top layer.
To frost the sides and top of the cake, I recommend that you take a quick look at some youtube videos. This one is very very direct and nice. But first of all, since this is a simple, homey cake, I recommend a simple, homey approach to the frosting. This frees you to understand this as a simple process. The goal is to evenly coat the sides and top, avoid bald spots, and grout up the gap between the layers. Don’t overthink it. Use the edge of your offset spatula, use a metal ruler, use a metal frying pan spatula, use whatever implement you think can be pressed into service. Then, if you like, while the frosting is still fresh, you may add some patterns—for instance, swirling with the edge of the spatula using a fork or spoon or a spatula.