Hot Fudge sauce- Combine the sugar, cocoa, cornstarch, and salt. Add the water and stir until the mixture is smooth. Microwave on high for 1 1/2 minutes, stirring after 45 seconds. Add the butter, stir in and microwave for another 30 minutes. Stir in the vanilla. Let cool slightly.
Chocolate whipped cream - In a large bowl add the heavy whipping cream. Beat with an electric mix or immersion blender until soft peak begins to form, add the powdered sugar and cocoa powder gradually until you have the correct taste. Be sure not to over beat.
To assemble- Cut brownies into small bite size pieces. Add brownies to a bowl or glass, spoon the hot fudge over top of the brownie bites, spoon the chocolate whipped cream over top of the hot fudge sauce. Repeat.