Instructions:

For the Cake:
Preheat the oven to 350°F (175°C). Grease and line two 9-inch (23cm) round cake pans.
In a large bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the buttermilk, vegetable oil, eggs, and vanilla extract. Mix until combined.
Slowly stir in the boiling water (the batter will be thin).
Divide the batter evenly between the prepared pans and bake for 30–35 minutes or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
For the Frosting:
Beat the butter in a large bowl until creamy.
Add powdered sugar, cocoa powder, and salt. Beat on low speed until incorporated.
Add vanilla and heavy cream, then beat on high speed for 2-3 minutes until fluffy.
Assembly:
Once the cakes are completely cool, slice each cake horizontally to create 4 layers in total.
Place the first layer on a serving plate and spread a thin layer of frosting. Repeat for all layers.
Cover the outside of the cake with frosting and smooth it out.
Optional:

Drizzle melted chocolate or ganache over the top for extra richness.
Enjoy your decadent chocolate cake!