Chocolate refrigerator cake

Another old-fashioned recipe I've had in my little black notebook for so long that I can't remember whether it came from a magazine, a friend, or the back of a box. Serves 8-10.

2-1/2 dozen ladyfingers
4 squares unsweetened chocolate
1/2 cup granulated sugar
1/4 cup water
4 egg yolks, lightly beaten
1/2 cup unsalted butter
1 cup confectioners sugar
4 egg whites, stiffly beaten
1 cup whipping cream, beaten (for topping)

Separate ladyfingers and line bottom and sides of a 9x12-inch glass baking dish with half of them. Put chocolate, granulated sugar and water into a double boiler over low heat. Stir until chocolate and sugar are melted, and the mixture is smooth. Whisk in the egg yolks gradually; continue to cook mixture until thick and smooth, stirring constantly. Remove from heat and let cool. In a large bowl, cream the butter and confectioners sugar. Add the cooked mixture, and then the stiffly beaten egg whites. Pour the chocolate mixture into the lined glass baking dish, and then cover with another layer of ladyfingers. Put in the refrigerator or freezer and let set overnight or up to 24 hours. When ready to serve, cut into single-serving squares and add a dollop of whipped cream.