• Start by preheating the oven to 350°F (175°C) and prepare your cake pans with parchment paper and grease them.

For Cake:

• In a large mixing bowl, combine sugar, flour, cocoa powder, baking powder, and salt. Use either a stand mixer or a hand mixer, and mix until everything is well mixed.
• Gradually incorporate the room temperature butter into the dry mixture while mixing at a low speed until there are no visible chunks of butter and the mix appears like damp sand.
• Add the egg whites and mix on low speed until just combined. Then pour in the buttermilk in two separate additions, blending well after each.
• Incorporate vegetable oil, vanilla extract, and optional espresso into the mixture. Mix on low until everything is smoothly combined.
• Scrape the bowl's sides and increase the mixer speed to medium for about a minute to blend everything thoroughly and lighten the batter.
• Evenly distribute the batter among the prepared pans, ensuring uniformity which can be assisted by using a kitchen scale.
• Bake for 33-35 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs attached.
• Allow the cakes to cool in the pans for about 10 minutes. Then, using a small spatula, gently loosen the edges and remove the layers carefully.
• To speed up cooling, place the cake layers into the freezer for 30 minutes. Once cooled, level the tops with a serrated knife if necessary.

For Frosting:

• Beat the room temperature butter in a bowl on high speed until smooth and creamy.
• Add the sifted cocoa powder, vanilla extract, and salt, mixing on low until everything is well blended.
• Gradually incorporate the powdered sugar. Around halfway, pour in the heavy cream to help with the mixing process. Scrape the sides of the bowl as needed.
• Finally, fold in the melted and cooled dark chocolate until you achieve your desired frosting consistency.
• For a silky texture, gently stir the frosting by hand with a rubber spatula for a couple of minutes to eliminate any air bubbles.
• On a cake board, layer and frost the cooled cake with frosting between each layer. Start with a small amount of frosting to hold the first layer in place.
• Apply an even layer of frosting between each cake layer, and then cover the outside with a thin layer. Chill until the frosting between layers firms up, about 30 minutes in the refrigerator or 10 in the freezer.
• Once set, add a thicker coat of frosting on top and smooth it out. Feel free to decorate as desired and savor your delicious cake!