Preheat oven to 350 degrees. Grease a 9-inch square baking pan.

In a large bowl, combine flour, cocoa, and salt. Melt butter in a large saucepan and remove from heat. Stir in sugar and vanilla. Quickly whisk in eggs, one at a time.

Add butter mixture to flour mixture and stir until well blended. Spread brownie batter into prepared pan and bake for 25-30 minutes. Cool completely.

Meanwhile, make the mousse by heating marshmallows, chocolate and milk in a medium saucepan over medium-low heat, stirring often until completely smooth. Cool completely.

With an electric mixer, beat whipping cream until stiff peaks form. Stir in cooled chocolate mixture.

Spread mousse evenly over cooled brownies. Drizzle with additional melted chocolate, or top with chocolate shavings, if desired.

Refrigerate brownies for at least 30 minutes, or until ready to serve. Keep leftover brownies in the refrigerator (this will also make the brownie layer dense and chewy!).