1. Preheat the oven to 350°F (177°C). Grease three 8-inch cake pans, line with parchment rounds, then grease the parchment.
   
2. In a large bowl whisk together the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder.
   
3. In another bowl whisk the oil, eggs, and vanilla until smooth. Add the buttermilk and mix to combine.
   
4. Pour the wet mixture into the dry ingredients. Add the hot coffee and whisk until fully combined. The batter will be thin.
   
5. Divide the batter evenly between the 3 pans. Bake for 22–26 minutes, or until a toothpick inserted in the center comes out clean.
   
6. Cool the cakes in the pans on a wire rack until completely cooled.
   

Make the Frosting

1. Beat the butter with a mixer on medium speed until smooth and fluffy, about 2 minutes.
   
2. Add the powdered sugar and cocoa powder, mix on low until combined.
   
3. Add the sour cream, hot water, vanilla, and salt. Beat until creamy and smooth.
   
4. Pour in the melted chocolate (lukewarm, not hot) and beat until silky and spreadable.
   

Assemble and Frost

1. Level the cakes if domed.
   
2. Place one cake layer on a stand or plate, spread frosting evenly on top.
   
3. Add the second layer, more frosting, then the third layer.
   
4. Spread the remaining frosting over the top and sides of the cake.
   
5. Refrigerate for 30–60 minutes before slicing to help the cake set.