Break up the chocolate into pieces and put into the pot with the coffee. Heat on low, just until the chocolate melts and can be stirred together. Add the butter and whisk it in as it melts. Beat in the egg yolks one by one. Set aside.
Bring the chilled bowl out and pour in 1 cup of the heavy cream. Whisk the cream until it is soft and clings to the whisk, but not stiff. Set aside.
In the bowl of the stand mixer (or in large bowl with hand mixer) beat the egg whites until you have soft peaks. Continue to beat while adding the confectioners sugar a spoonful at a time, until you have stiff shiny peaks.
Drizzle the chocolate down the sides of the bowl with the egg whites and fold them together. When they are almost blended, fold in the whipped cream.
Spoon into servings bowls or martini glasses. Cover and chill for several hours.
WHIPPED CREAM
Chill the clean mixing bowl again. Pour in the remaining 1 cup of heavy cream. Whisk until you have soft peaks. Pipe through a pastry bag or dollop with a spoon onto the chilled mousse and serve.