Combine chocolate, sugar, milk and salt in top of double boiler.
Heat and stir until blended.
Add egg yolks, and cook, stirring, 3 minutes.
Remove from heat, and add vanilla.
Stand in bowl of ice cubes, and stir occasionally until cool and thick.
Fold in stiffly beaten cream and egg whites.
Chill to spread easily.For best results use a bright, shiny baking sheet to reflect heat.
Darkened metal lets cookies brown too quickly.
Grease sheets with shortening to prevent sticking.
Place baking sheet in oven so that 2 inches of oven rack show all around it, for proper circulation of heat.
The oven rack should be placed almost in center of oven.
If 2 racks are used, divide oven into thirds, and stagger baking sheets so they are not directly above one another.
When 2 baking times are listed, always check to see if cookies are done at shortest time given.
Let delicate cookies cool slightly before lifting, and cool them thoroughly on cake rack.
Soft cookies keep best in airtight containers.
Crisp ones need a little air circulation around them and are thus best stored in loosely covered containers.
Unbaked cookie doughs freeze well as do baked cookies if properly packed.