Start with the ganache frosting. Add heavy cream and chopped chocolate to a microwave-safe bowl. Microwave in 15–20 second intervals, stirring between each, until mostly melted and smooth.
Cover and let the ganache sit at room temperature for 1½–2 hours, stirring every 30 minutes, until thick and spreadable.
Preheat the oven to 350°F. Grease and line two 8-inch or 9-inch cake pans with parchment paper.
If making homemade buttermilk, combine 1 cup milk with 1 tablespoon vinegar and let sit for a few minutes.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Sift flour if needed to remove lumps.
Add eggs, vegetable oil, and buttermilk. Whisk gently until smooth and combined.
In a separate bowl, whisk boiling water with cocoa powder and instant coffee until glossy and smooth.
Fold the cocoa mixture into the batter until fully combined. Do not overmix. The batter will be thin.
Divide the batter evenly between the prepared cake pans.
Bake for 20–25 minutes for 9-inch pans or 23–28 minutes for 8-inch pans, until a toothpick inserted into the center comes out with moist crumbs.
Let cakes cool in pans for 10–15 minutes, then transfer to a wire rack to cool completely.
To assemble, level cake tops if needed. Place one cake layer on a serving plate and spread 1/3 of the ganache on top.
Add the second cake layer and frost the top and sides with remaining ganache. Create a rustic finish with an offset spatula if desired.
Slice and serve.