Equipment: An ice water bath An ice cream machine 20 clamshells, such as Cherrystones A small ice-cream scoop
At least 1 day before serving, start the ice cream: Heat the coffee beans, half-and-half, heavy cream, and vanilla bean in a saucepan over medium heat just to a boil, stirring occasionally to make sure the mixture doesn't burn or stick to the bottom of the pan. Turn off the heat and let cool. Cover and refrigerate overnight to infuse the coffee flavor.
The next day, whisk the egg yolks and sugar together in a bowl. Strain the vanilla and coffee beans out of the cream mixture and re-warm it in a heavy saucepan. When the cream mixture reaches a fast simmer (do not let it boil), turn off the heat. In a thin stream, whisk half of the cream mixture into the egg yolk mixture, then pour in the remaining half and whisk well.
Pour the mixture back into the saucepan. Cook over medium heat, stirring constantly with a wooden spoon. At 160 degrees F, the mixture will give off a puff of steam. When the mixture reaches 180 degrees it will be thickened and creamy, like eggnog. Test it by dipping a wooden spoon into the mixture. Run your finger down the back of the spoon. If the stripe remains clear, the mixture is ready; if the edges blur, the mixture is not quite thick enough yet. When it is ready, quickly remove it from the heat.
Pour the cream mixture through a fine sieve into a bowl. Rest the bottom of the bowl in the ice bath. Let the mixture cool for 2 hours, stirring occasionally. Then freeze according to the directions of your ice cream machine.
Chocolate Shells: Cut pieces of aluminum foil large enough to more than cover the back side of each clam shell. Cover the back (outside) of each shell with foil, folding it over the rim to hook it tightly to the shell. Press the foil against the surface, rubbing to smooth it so that it acts as a skin for the shell. Arrange the wrapped shells on a pan, with the wrapped sides facing up. Melt the chocolate. Spoon 1 to 2 tablespoons of chocolate onto the wrapped side of one shell, spreading it with the back of the spoon to cover the shell all the way to the edge. As needed, add more chocolate until the entire back of the shell is well coated with chocolate. Repeat with the remaining shells and chocolate. Refrigerate until set, about 20 to 30 minutes.
Once the chocolate is set, carefully unhook the foil from all around the rim of the shell. Lift the real shell out of the foil, leaving you with the chocolate-covered foil cups. Place the shells back in the refrigerator for 15 minutes to re-harden. Taking the shells out of the refrigerator one at a time, carefully peel the foil back to reveal the chocolate shell. Repeat with the remaining shells. Store in an airtight container until ready to serve. (The shells can be made up to 1 week in advance.) To serve, pour raw sugar on a platter to look like sand. Place a scoop of ice cream in each clam shell and rest it on the sand. Serve immediately.