Prepare white cake as directed on the box, and pour into two 9-inch round cake pans. Allow cakes to cool to room temperature.
Prepare brownies as directed, baking in a 9-inch round cake pan.
To make the chocolate sauce, place the chocolate, sweetened condensed milk and 1 tablespoon water in a small saucepan and cook over low heat, stirring until the chocolate is melted (about 5 minutes).
While the brownies are still warm, place the brownie layer on a cake stand. Slowly pour the chocolate sauce on top, letting it soak into the brownie. Allow the sauce to soak in for at least 30 minutes. Meanwhile, make the cream cheese frosting (see below).
Place a white cake layer on top of the brownies and top with approximately ¼ cup of the frosting using an offset spatula.
Add the second white cake layer and cover the cake in a layer of cream cheese frosting. Since I'm not a pro at decorating, but did the best I could.
Chill the cake until ready to serve.
To make the frosting:
Combine the cream cheese, butter and vanilla in a stand mixer and mix on medium speed until incorporated, about 1 or 2 minutes. Add the confectioner's sugar 1 cup at a time, until the frosting is light and creamy. For thicker/stiffer frosting, add more confectioner's sugar ¼ cup at a time. If the frosting is too stiff, add milk 1 tablespoon at a time.