Preheat the oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper.

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, mix the oil, eggs, and vanilla together on medium-high speed until combined. Add the buttermilk and mix until combined.

Pour the dry ingredients into the wet ingredients, add the hot coffee/water, and whisk or beat on low speed until the batter is completely combined.

Divide the batter evenly between the prepared pans. Bake for 23–26 minutes or until a toothpick inserted in the center comes out clean.

Cool the cakes in the pans set on a cooling rack for 1 hour. Run a knife around the edges to help loosen the sides, remove the cakes from the pans, peel off the parchment, and place on the cooling rack to finish cooling.

Make the buttercream: Beat the butter on medium speed until creamy—about 2 minutes. Add confectioners’ sugar, cocoa powder, 3 Tablespoons heavy cream, vanilla extract, and salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute.

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.

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.

Refrigerate uncovered cake for at least 30–60 minutes before slicing to help set the shape.