Preheat oven to 350 degrees F.

In a saucepan, combine the sugar and water and cook over medium heat until the mixture registers 220 degrees F on a candy thermometer.

In another saucepan, heat the milk until it is very hot and small bubbles form around the edge of the pan. Do not let the milk boil. Put the chocolate in a heatproof bowl and pour the hot milk over it. Stir until the chocolate melts and the mixture is smooth.

Return the chocolate mixture to the saucepan and carefully add the hot sugar syrup, using thick oven mitts or potholders. Heat the mixture gently over medium heat, stirring until smooth. Remove the pan from heat.

In a large bowl, whisk the egg yolks. Slowly drizzle about 1 cup of the hot chocolate mixture into the eggs, whisking well to temper. Pour the tempered yolks into the chocolate mixture and whisk. Strain through a fine-mesh sieve into a bowl. Stir in the espresso and set aside to cool slightly.

Pour the chocolate mixture into 8 (5-ounce) ramekins or custard cups. Set the cups in a large shallow roasting pan and set the pan on the center rack of the oven. Pour enough hot water into the roasting pan and come about halfway up the side of the ramekins. Bake for about 1 hour, or until the custards are set. Remove and let cool in the water bath. Remove ramekins from water bath, cover with plastic wrap and refrigerate until ready to serve. This recipe can be prepared up to 2 days in advance and kept cool in a refrigerator.