Make the cake: Preheat oven to 350 degrees. Butter and lightly flour two 9-inch round cake pans. In a bowl, whisk flour, cocoa, baking soda, and salt.
In a mixing bowl, cream butter and sugars until fluffy. Add eggs, one at a time, beating until well incorporated. Beat in vanilla.
Add the flour mixture to the butter mixture in three additions, alternating with buttermilk and beginning and ending with the flour mixture. Scrape bowl as needed.
Pour batter into pans. Bake until toothpick inserted in center comes out clean, 25 to 30 minutes. Cool in pans 10 minutes before inverting cakes onto rack to cool completely.
Make the buttercream: In mixing bowl, beat butter and marshmallow cream until fluffy. Beat in cocoa, chocolate, vanilla, and 1 tablespoon water. If too thin, refrigerate about 5 minutes.
Place one layer on platter. Spread top with 1 cup buttercream. Place second layer on top. Frost entire cake with remaining buttercream.
To melt chocolate, heat in the microwave for 15-second intervals until almost melted, then stir to finish melting. The cake can be frosted in advance and refrigerated.