Place chocolate, butter, and hot water in a large bowl. Microwave in 15-second bursts, stirring in between, until chocolate is melted and smooth. Let cool until it feels just slightly warm.
While chocolate is cooling, whip cream in the bowl of a stand mixer fitted with the whisk attachment until stiff peaks form. Transfer to a bowl and refrigerate. Thoroughly clean mixer bowl and attachment.
Once chocolate has cooled, stir in egg yolks.
Whip egg whites in the bowl of a stand mixer fitted with the whisk attachment. When foamy, slowly drizzle in sugar and continue whipping on high until soft peaks form.
Stir approximately 1/4 of the whipped cream into the chocolate mixture.
In two additions, fold the egg whites into the chocolate mixture.
Fold in remaining whipped cream.
Transfer to serving dishes and refrigerate for at least 8 hours before serving