Ingredients
Method
- Preheat your oven to 350°F. Grease a 9×13 baking pan or two 8-inch round cake pans and set aside.
- In a large bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, and salt until evenly combined.
- In a separate bowl, whisk the sourdough discard, milk (or buttermilk), oil, eggs, and vanilla extract- making sure to mix well. (making sure eggs are room temp as well as discard)
- Pour the dry ingredients into the bowl with the wet ingredients and mix just until combined. Slowly add the hot water, stirring until the batter is smooth. The batter will be fairly thin, which is exactly what you want for a moist cake.
- Pour the batter into the prepared pan and bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely before frosting.
- While the cake is cooling, place the softened butter in a large mixing bowl. Beat with a mixer for 2–3 minutes until light and creamy.
- Mix in the cocoa powder until fully combined with the butter.
- Gradually add the powdered sugar, mixing on low speed so it blends smoothly into the frosting.
- Add the heavy cream, vanilla extract, and salt. Beat the frosting for another 2–3 minutes until it becomes light, fluffy, and easy to spread. If it feels too thick, add a small splash of cream. If it’s too soft, mix in a little more powdered sugar.
- Once the cake is completely cool, spread the chocolate buttercream evenly over the top.Slice and serve.
Notes
Discard age matters. Use discard that's been in your fridge for up to a week. Older discard (10+ days) develops a sharper acetic-acid tang that can come through in the final cake. If your discard smells like nail polish remover, swap it out for a fresher batch.
Room-temperature ingredients are non-negotiable. Cold eggs and cold discard cause the batter to seize and bake unevenly. Pull the eggs, discard, and milk out of the fridge 20–30 minutes before you start. If you forget, drop the cold eggs into a bowl of warm water for 5 minutes.
Spoon and level your flour. Scooping the cup directly into the flour packs it, which is the #1 cause of a dry, dense chocolate cake. For best results, weigh it (220g for 1¾ cups). The difference between great and just-okay almost always traces back to this step.
The water must be hot. Lukewarm water won't bloom the cocoa the same way, and blooming the cocoa is what unlocks the deep chocolate flavor. This is the most important non-obvious step in the recipe.
The batter will look alarmingly thin. That's the whole point. Don't add more flour — thin batter is exactly what produces the moist, tender crumb this cake is known for. Pour it in and trust it.
Don't overmix. Once the wet meets the dry, stir just until combined. A few small lumps are fine. Overmixing develops gluten and gives you a tough, chewy cake instead of a tender one.
Test early, especially for round pans. Start checking at 28 minutes for a 9×13 and 25 minutes for two 8-inch rounds. The difference between perfectly moist and overbaked is about 4 minutes — a clean toothpick with one or two moist crumbs clinging is exactly right.
Cool completely before frosting. Warm cake plus buttercream equals melting, sliding, sad cake. Wait until the cake is fully room temperature, or speed it up with 30 minutes in the fridge.
For the frosting: Soften the butter properly (gives when pressed, still holds shape — not melted, not cold). Sift the cocoa to avoid lumps. Beat the finished frosting for 2–3 full minutes — that's what takes it from heavy to fluffy. A pinch of salt at the end keeps it from tasting flat.
Pan flexibility. This batter works in a 9×13, two 8-inch rounds, 24 cupcakes (350°F, 18–22 min), or a Bundt pan. Adjust bake time and check with a toothpick.
Coffee swap. Substitute hot brewed coffee for the hot water for even deeper chocolate flavor — it doesn't make the cake taste like coffee, it just makes the chocolate taste more like chocolate. Same amount, same temperature.