Preheat the oven to 350 degrees F. Line the bottoms of two 9-inch cake pans with circles of was paper and then grease the pans with cooking spray.
In a medium bowl, pour the boiling water over the 6 oz. of semisweet chocolate. Set aside for a few minutes. Once the chocolate is thoroughly melted, add the cocoa and stir until smooth. Set aside to cool to room temperature.
In a small bowl, whisk together the flour, baking soda, and salt. In a large bowl with an electric mixer, cream the butter and brown sugar. Add eggs one at a time beating well after each addition. Beat in the vanilla. Add the flour mixture and half of the chocolate mixture. Beat on low speed to combine, and then on high for 1 1/2 minutes. Add the remaining chocolate mixture and beat till combined.
Pour the batter into the prepared cake pans and bake for 30-40 minutes or until a toothpick inserted into the center of the cake comes out clean. Allow the cake to cool in the pans for 20 minutes before removing (or they will fall apart). Invert the cakes onto wire racks to cool completely before assembling and frosting.
In a large bowl with an electric mixer, cream the butter. In a medium bowl, whisk together the sugar and cocoa. Beat 1/3 of the sugar-cocoa mixture into the butter. Mix in the vanilla. Add the rest of the sugar-cocoa mixture alternately with the milk and beat until frosting is smooth. Assemble the cake.