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Preheat the oven to 325°F. Lightly grease two 6" round pans that are at least 2" deep.
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To make the cake: In a large bowl, sift together the flour, sugar, cocoa, baking soda, baking powder, and salt. Whisk the dry mixture until the cocoa is evenly distributed.
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In a small bowl, whisk together the water, buttermilk, egg, oil, and vanilla until combined.
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Add the wet ingredients to the dry ingredients and whisk slowly to incorporate, then vigorously until smooth. The batter will be thick.
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Divide the batter evenly between the pans. You’ll use about 1 cup (290g) per pan.
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Bake the cakes for 30 to 35 minutes, until a toothpick or thin knife inserted into the center comes out clean, or with a few moist crumbs clinging to it.
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Allow the cakes to cool for 5 to 10 minutes in the pan before turning them out onto a wire rack to cool completely. Cake layers can be wrapped and frozen for up to a month before frosting.
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To assemble the small chocolate cake: When the cake layers are completely cool, level them by cutting the dome off each layer, if necessary.
To make a 4-layer cake, use a large, serrated knife to cut each layer in half horizontally. Frost the top of one layer (use 1/2 cup or about 100g of frosting between each layer), then top with the second layer and repeat until all the layers are used. Finally, frost the top and sides of the cake. You’ll use an entire batch of Chocolate Ermine Icing (about 2 1/2 cups), with none to spare.
To make a 2-layer cake, use a scant 3/4 cup (about 140g) frosting between the two layers of cake and use the rest to frost the top and sides of the cake. You may have some frosting leftover, depending on how you decorate the cake. See our blog post, How to assemble and frost a layer cake, for details.
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Storage information: Store the cake layers, well wrapped, in the refrigerator for up to 3 days; freeze for longer storage. Store the assembled cake in the refrigerator, covered, for up to 5 days or freeze for longer storage.
