Every fall, I pick the Concord grapes that grow all over Block Island, in New England, and make jelly. Inspired by that ritual, this dessert contrasts sweet, tart grape jelly with smooth, creamy buttermilk panna cotta. The differing textures play off each other beautifully, and both melt in your mouth. Here, I cheat and use bottled grape juice; the flavor is just as intense, and my hands aren't stained purple.