Preheat oven to 300 degrees. Bring half-and-half just to a simmer in a small saucepan. Remove from heat; add chocolate and sugar. Let stand 5 minutes.
In a large bowl, stir together yolks, vanilla, cocoa, salt, and liqueur if desired. With a fork, stir chocolate mixture until smooth, then gradually stir into yolk mixture. Pour through a fine sieve into a glass measuring cup.
Place 4 custard cups or ramekins (3 to 4 ounces each) in a shallow roasting pan, and divide chocolate mixture among them. Pour hot water into the pan so it reaches halfway up the sides of cups.
Bake until custards are almost set in centers, about 30 minutes (custards will firm as they cool). Carefully remove cups from hot-water bath; let cool slightly.
Cover with plastic wrap; refrigerate. Just before serving, dollop with whipped cream, if desired.
Pots de creme can be made two days ahead and refrigerated.