Prepare brownies according to package directions in a 9x13 inch baking pan. Bake and allow to cool completely at room temperature, then refrigerate for 30 minutes until firm.

Beat softened cream cheese with powdered sugar and vanilla extract until smooth and fluffy, about 3 minutes. Spread this mixture evenly over the cooled brownie base and refrigerate for 20 minutes to set.

Whip 1 cup of heavy cream with cocoa powder and granulated sugar until stiff peaks form. Gently spread this chocolate mousse over the cream cheese layer and refrigerate for another 30 minutes.

Whip the remaining 1 cup of heavy cream with 2 tablespoons powdered sugar until stiff peaks form. Pipe or spread whipped cream decoratively over the chocolate mousse layer.

Melt chocolate chips in 20-second intervals in the microwave, stirring between each interval until smooth. Drizzle melted chocolate over the whipped cream topping and sprinkle with chocolate shavings.

Refrigerate for at least 3 hours, preferably overnight before serving.