Preheat oven to 300 degrees. Bring half-and-half and cream just to a simmer in a small saucepan. Remove from heat; add both chocolates and sugar. Let stand 5 minutes.

In a large bowl, stir together yolks, vanilla, cocoa, cayenne, 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 6 custard cups or ramekins 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 (or at least touches the bottom of the mug if using glass hot chocolate mugs)

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, top with a large marshmallow and brown off under the broiler or using a mini kitchen torch.

Crush a few freeze-dried raspberries over the top.

Serve immediately while marshmallow is still warm.