Position racks in the upper and lower thirds of the oven; preheat to 250 degrees F. Line 2 baking sheets with parchment paper. Beat the egg whites and cream of tartar in a large bowl with a mixer on medium speed until white and foamy. Gradually beat in the sugar until thick and glossy, about 5 minutes, then beat in the vanilla.
Transfer the meringue to a pastry bag fitted with a 3/8-inch round tip. Pipe 3-inch bones onto the prepared baking sheets. Bake, switching the pans halfway through, until the meringues are dry and crisp, 1 1/2 to 2 hours. Transfer to a rack and let cool completely on the baking sheets.
Carefully loosen the meringue bones from the parchment paper and sandwich with a thin layer of jam. Spread or pipe a thin layer of jam on the flat side of a meringue; sandwich with another meringue. Repeat with the remaining meringues.
Photograph by Ralph Smith