For the mousse: In a stand mixer or large bowl with electric mixer, beat cream cheese until smooth. Add sugar and beat until combined.
Puree blackberries and press through a fine mesh strainer into the cream cheese. Beat until combined.
Fold in whipped cream, carefully turning the mixture over with a spatula until combined.
Refrigerate until you are ready to assemble.
For the pudding: Combine sugar, cocoa, corn starch and salt in a large pot. Slowly whisk in milk. Cook over medium heat stirring often. As mixture begins to thicken, whisk constantly.
Transfer to a bowl and press plastic wrap onto the top of the pudding. Set aside to come to room temperature(you can refrigerate to speed up the process!)
Assembly: Cut brownie cake into cubes. Layer first cake, then pudding, then mousse, creating as many layers as you desire. Top with whipped cream and fresh blackberries if desired.