Place chocolate in the top of a double boiler. Heat until chocolate is completely melted. Remove from heat
10 ounces bittersweet chocolate pieces
Mix in butter with one tablespoon at a time. Mix with a whisk until butter is completely combined.
2 tablespoons butter
Whip Egg Whites
Put egg whites in a bowl. Mix with a mixer at high speed until egg whites form soft peaks.
4 egg whites
With mixer running, slowing add sugar while mixing. If you add sugar too fast, your egg whites could deflate.
¼ cup granulated sugar
Whipped Cream
Put heavy cream in a different bowl. Mix with a mixer at high speed until soft peaks are formed.
1 cup heavy cream
Assemble Mousse
Add ⅓ of the egg whites into the chocolate mixture. Use a large spatula and fold egg whites into chocolate. Do not stir mixture or egg whites will deflate.
Add remaining egg whites and continue folding until no white remains.
Add whipped cream and fold gently until mousse does not show any white streaks and is one consistent color.
Place into individual serving dishes and serve or cover and refrigerate.
Notes
If serving later, put into individual serving dishes and cover each dish with plastic wrap before refrigerating. Garnish before serving with whipped cream and fresh berries.