Set a medium bowl in a large bowl of ice water. In another medium bowl, whisk the egg yolks with 1/2 cup of the sugar until pale, about 3 minutes.
In a medium saucepan, combine the cream, milk, salt and remaining 1/4 cup of sugar and bring to a simmer, whisking until the sugar is completely dissolved. Whisk the hot cream mixture into the beaten egg yolks in a thin stream.
Transfer the mixture to the saucepan and cook over moderately low heat, stirring constantly with a wooden spoon, until the custard is thick enough to lightly coat the back of the spoon, about 4 minutes; don't let it boil. Pour the custard through a fine-mesh strainer into the medium bowl in the ice water. Let cool completely, stirring frequently. Refrigerate the custard until very cold, at least 1 hour.
Pour the custard into an ice cream maker with flavorings, if using, and freeze according to the manufacturer's instructions. Transfer the frozen custard to a plastic container, cover and freeze until firm, at least 3 hours.
Add flavorings to the ice cream maker at the beginning of the freezing cycle. Chocolate-Toffee: 1 cup broken chocolate-and-toffee-covered saltines or pretzels Blackberry-Sage: 3/4 cup blackberry preserves mixed with 1 tablespoon minced fresh sage Salted Caramel: 3/4 cup pourable salted- caramel sauce Strawberry-Jalapeño:1 cup strawberries macerated in 3 tablespoons sugar with 1 teaspoon minced jalapeño