In a small heat proof bowl, melt 6 ounces of semi-sweet chocolate with a tablespoon of butter and whisk until smooth and fully combined. Whisk in a teaspoon of espresso powder. Set aside to cool. In a large bowl, whip 4 large egg whites with 1/3 cup of granulated sugar until stiff, glossy peaks have formed. Set aside.
In a separate bowl, whip 2/3 cup of heavy cream until the cream will hold its shape
Whisk the 4 egg yolks until very smooth and fold them into the whipped heavy cream
Add the slightly cooled melted chocolate and butter into the egg yolks and whipped heavy cream and gently fold until mostly combined
Add 1/3 of the stiff egg whites into the chocolate and heavy cream mixture and gentle fold to combine. Add the remaining egg whites and gently fold only until combined
Transfer the chocolate mousse into eight 6 ounce airtight container and chill for several hours before serving
Garnish with whipped cream and chopped chocolate