Step 1: Prepare and Preheat
Preheat your oven to 350°F (175°C). Grease three 8-inch round cake pans with butter or non-stick spray. Line the bottoms with parchment paper circles and flour the sides. This guarantees flawless release.

Step 2: Combine Dry Ingredients
In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer), sift together the sugar, flour, cocoa powder, baking soda, baking powder, and salt. Whisk on low speed for one minute to ensure absolute homogeneity and aeration.

Step 3: Add Wet Ingredients (Except Coffee)
To the dry ingredients, add the eggs, buttermilk, oil, vanilla extract, and orange zest. Mix on medium speed for exactly 2 minutes. The batter will be thick and pale. Scrape down the bowl thoroughly.

Step 4: Incorporate the Hot Liquid
With the mixer on low speed, carefully and slowly pour in the hot coffee or boiling water. The batter will be very thin—this is correct and essential for the moist, tender crumb. Mix just until combined, about 30 seconds. Do not overmix.

Step 5: Bake to Perfection
Divide the batter evenly between the prepared pans (using a kitchen scale is ideal for perfect layers). Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with just a few moist crumbs attached. The cakes will have just begun to pull away from the sides.

Step 6: Cool Completely
Let the cakes cool in their pans on a wire rack for 15 minutes. Then, run a thin knife around the edges, invert them onto the rack, peel off the parchment, and allow them to cool completely. Wrap each layer tightly in plastic wrap and refrigerate for at least 2 hours, or freeze for 1 hour. A cold cake is infinitely easier to frost without crumbs.

Step 7: Make the Swiss Meringue Buttercream
Combine egg whites and sugar in the heatproof bowl of a stand mixer. Place over a pot of simmering water (double boiler), ensuring the bowl doesn’t touch the water. Whisk constantly until the mixture reaches 160°F (71°C) and the sugar is fully dissolved (rub a bit between fingers; it should feel smooth). Attach the bowl to the mixer fitted with the whisk. Whip on high speed until a thick, glossy, cool meringue forms and the bowl is cool to the touch (10-15 minutes). Switch to the paddle attachment. With the mixer on medium-low, add the softened butter one cube at a time, waiting for each to incorporate. It may look curdled briefly; keep beating until it becomes smooth, thick, and creamy. Beat in the melted (and cooled) chocolate, sifted cocoa powder, vanilla, orange juice, zest, and salt until perfectly uniform and silky.