Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper for easy removal.
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the granulated sugar and eggs until well combined. Stir in the melted chocolate mixture and vanilla extract.
Sift in the flour and salt, folding gently until just combined. Pour the brownie batter into the prepared baking pan and spread evenly.
Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let the brownies cool completely in the pan on a wire rack.
While the brownies are cooling, prepare the chocolate mousse. In a mixing bowl, whip the heavy cream with powdered sugar and instant coffee granules (if using) until soft peaks form.
Gently fold in the vanilla extract. Once the brownies are completely cool, spread the mousse evenly over the top of the brownies.
Chill the brownies in the refrigerator for at least 2 hours to allow the mousse to set.
Before serving, garnish with chocolate shavings or a dusting of cocoa powder if desired. Cut into squares and enjoy!