Carolyn Snyder, owner of Round Swamp Farm in East Hampton, has been making jellies, jams, and preserves for over 30 years, amending her recipes regularly. To make beach plum jelly, she suggests starting with a grape jelly recipe—like the one on boxes of liquid pectin (though she recommends using about 1 cup less sugar than the instructions call for). Snyder likes to use a combination of ripe and unripe beach plums to give her jelly the characteristic beach plum tartness. She makes a juice from the plums by placing them in a pot with just enough water so they don’t scorch (about 2 1/2 cups for 3 lbs. of beach plums) and lightly crushing them as they come to a boil. Snyder reduces the heat, covers the pot, and simmers the plums, stirring frequently, until they are soft, about 30 minutes. Then she sets the mixture to drip in a jelly bag or a colander lined with cheesecloth. Snyder next gently presses the juice from the pulp, then discards the pulp and cooks the juice with sugar and pectin according to her recipe. She stirs her jelly constantly as it cooks. (Stirring, she says, keeps all the flavor in the jelly rather than allowing it to escape in a foam that must be skimmed.) And she cautions that no two batches of beach plums—and thus no two batches of beach plum jelly—are ever alike.