Combine 4 (1-ounce) squares semisweet chocolate and 1/2 cup whipping cream in a small heavy saucepan; cook over low heat, stirring constantly until chocolate melts. Add remaining 1 1/2 cups whipping cream, egg yolks, 1/2 cup sugar, and vanilla, string with a wire whisk until sugar dissolves and mixture is smooth. Pour mixture evenly into 5 (5- x 1-inch) round individual baking dishes; place dishes in a large roasting pan or a 15- x 10- x 1-inch jellyroll pan. Prepare 1/2-inch water bath (see Brûlée Basics below).
Bake at 275° for 55 minutes or until almost set. Cool custards in water in pan on a wire rack. Remove from pan; cover and refrigerate 8 hours or overnight.
Sprinkle about 1 1/2 tablespoons brown sugar evenly over each custard; place custards in jellyroll pan.
Broil 5 inches from heat (with electric oven door partially opened) until brown sugar melts. Let stand 5 minutes to allow sugar to harden. Garnish, if desired.
Note: Baking time is for 5- x 1-inch round individual baking dishes. As a general rule, to use 4-, 6-, or 8-ounce custard cups, bake for an additional 15 to 20 minutes. When the crème brûlée is done, the center will still be slightly liquid and a knife will not come out clean. The yield will vary with different size dishes: For 4-ounce cups you'll get 10 servings, for 6-ounce cups you'll get 7 servings, and for 8-ounce cups you'll get 4 servings.
To make a Chocolate-Raspberry version, place 8 to 10 fresh raspberries in each baking dish, add chocolate custard, and increase baking time to 1 hour and 5 minutes.
Don't panic when you see the term water bath. A water bath is simply a roasting pan or jellyroll pan filled with water. The water creates a cushion from the heat of the oven, allowing the custards to bake slowly without curdling.
Don't burn yourself. Before you take the water bath out of the oven, remove some of the water with a basting bulb or a long-handled ladle.
When you broil the brown sugar, get the crème brûlées as close to the heating element as possible. To do this, place an inverted roasting pan on the top shelf of the oven; then place the crème brûlées on a baking sheet on top of the roasting pan.
An adventurous alternative to the broiler is a welding torch. Your dinner guests will think you have gone mad, but the torch gives the ultimate glassy crust. Torching is the professional chef's method of choice.
Crème brûlée is an even more extraordinary dessert if the custard is cold and firm when you crack into the warm caramelized sugar topping. Here's the secret: Place the custards in a roasting pan filled with ice, and then broil them. The ice keeps the custards cold while the sugar melts.
You can bake the crème brûlées ahead of time, but wait until just a few minutes before serving to caramelize the sugar. The caramelized sugar will begin to liquefy if the custards sit for more than an hour.
We found that Dixie Crystals brand brown sugar works best for crème brûlées. It caramelizes evenly to a perfect golden brown.
Don't waste your money on salamanders sold in gourmet catalogs and used for caramelizing the top of crème brûlées. They work like branding irons to melt the brown sugar. When we tried one, we ended up with burnt--not caramelized--sugar.