Put the butter in a medium dish that can go in the microwave. Small chocolate chunks should be broken into the bowl. Stir between each 20-second burst of microwave heat until the chocolate is about 75% dissolved. Stirring will help the remaining heat in the bowl fully melt the chocolate. After allowing the mixture to cool for a few minutes, add the egg whites one at a time, whisking well after each addition. Place aside.

The egg whites should be foamy when they have been beat on medium-high speed in a stand mixer or electric hand mixer dish. Beat while adding the cream of tartar until soft peaks appear. Fold the egg white combination into the chocolate mixture until it is thoroughly combined using a large rubber spatula. Place aside.

Put the butter in a medium dish that can go in the microwave. Small chocolate chunks should be broken into the bowl. Stir between each 20-second burst of microwave heat until the chocolate is about 75% dissolved. Stirring will help the remaining heat in the bowl fully melt the chocolate. After allowing the mixture to cool for a few minutes, add the egg whites one at a time, whisking well after each addition. Place aside.

The egg whites should be foamy when they have been beat on medium-high speed in a stand mixer or electric hand mixer dish. Beat while adding the cream of tartar until soft peaks appear. Fold the egg white combination into the chocolate mixture until it is thoroughly combined using a large rubber spatula. Place aside.

Mousse can be made up to 1 day ahead of time. Cover with plastic wrap and keep chilled in the refrigerator. Add whipped cream topping and chocolate shavings up to a few hours before serving.