- Preheat oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)
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Make the cake: Whisk the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder (if using) together in a large bowl. Set aside. Using a handheld or stand mixer fitted with a whisk attachment (or you can use a whisk) mix the oil, eggs, and vanilla together on medium-high speed until combined. Add the buttermilk and mix until combined. Pour the wet ingredients into the dry ingredients, add the hot water/coffee, and whisk or beat on low speed until the batter is completely combined. Batter is thin.
- Divide batter evenly between pans. Bake for 23-26 minutes or until a toothpick inserted in the center comes out clean. Baking times vary, so keep an eye on yours. The cakes are done when a toothpick inserted in the center comes out clean. (Note: Even if they’re completely done, the cooled cakes may *slightly* sink in the center. Cocoa powder is simply not as structurally strong as all-purpose flour and can’t hold up to all the moisture necessary to make a moist tasting chocolate cake. It’s normal!)
- Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pan.
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Make the buttercream: With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy – about 2 minutes. Add confectioners’ sugar, cocoa powder, 3 Tablespoons heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Do not over-whip. Add 1/4 cup more confectioners’ sugar or cocoa powder if frosting is too thin or 1-2 more Tablespoons of cream if frosting is too thick. (I usually add 1 more.) Taste. Add another pinch of salt if desired.
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Assemble and frost: If cooled cakes are domed on top, use a large serrated knife to slice a thin layer off the tops to create a flat surface. This is called “leveling” the cakes. Discard or crumble over finished cake. Place 1 cake layer on your cake stand or serving plate. Evenly cover the top with frosting. Top with 2nd layer and spread remaining frosting all over the top and sides. I always use an icing spatula and bench scraper for the frosting. Garnish with chocolate chips, if desired.
- Refrigerate cake for at least 30-60 minutes before slicing. This helps the cake hold its shape when cutting.
- Cover leftover cake tightly and store in the refrigerator for up to 5 days.
Buttermilk: Buttermilk is required for this recipe. You can make your own DIY version of buttermilk if needed. Add 2 teaspoons of white vinegar or lemon juice to a liquid measuring cup. Then add enough room temperature whole milk to the same measuring cup until it reaches 1 cup. (In a pinch, lower fat or nondairy milks work for this soured milk, but the cake won’t taste as moist or rich.) Stir it around and let sit for 5 minutes. The homemade “buttermilk” will be somewhat curdled and ready to use in the recipe.