Preheat oven to 325°. Grease two 8" round cake pans with 1 tbsp. of the butter and set aside. Stir together the baking soda and 1⁄4 cup boiling water in a small bowl and set aside.
Whisk together flour and salt in a medium bowl and set aside. Combine 8 tbsp. of the butter and the brown sugar in a large bowl and beat with an electric mixer until fluffy. Add the eggs one at a time, beating briefly after each addition. Working in 3 batches, alternately add the flour mixture and buttermilk, beating briefly after each addition. Add the baking soda mixture (stir before adding) and chocolate and stir to make a smooth batter.
Divide the batter between prepared pans and bake until a toothpick inserted in the middle comes out clean, 35-40 minutes. Set the cake pans on a rack to let cool.
While the cakes are cooling, make the icing. Melt the remaining 6 tbsp. butter and transfer it to a large bowl. Add the confectioners' sugar, heavy cream, cocoa, and vanilla and beat until well combined and fluffy, about 2 minutes. Set aside.
Loosen the cakes from their pans. Place 1 cake on a large plate and spread top evenly with about 1 cup of the icing. Top with the second cake and use the remaining icing to spread over the top and sides. Serve immediately or refrigerate until ready to eat.