Chop the chocolate into very, very small pieces.
Add the butter, chopped chocolate, mini marshmallows and water to a large heavy bottom saucepan.
Turn the burner to low heat and gently stir as the mixture starts to melt. Be very careful that the chocolate doesn't burn.
Once completely melted, remove from the heat. Whisk vigorously if the butter seems to be separating.
In a separate large bowl, beat the heavy cream until stiff peaks form.
Gently fold the whipped cream into the cooled chocolate mixture using a rubber spatula until the mixture is no longer streaky. Go gently to keep the volume from the whipped cream.
Spoon the chocolate mousse filling into the pie plate and smooth the top. Cover the top of the pie plate, and place in the fridge to set for at least 4 hours.