Preheat oven to 120 C / Gas 1/2. Line 2 baking trays with foil and lightly grease the foil.
Beat egg whites with cream of tartar and salt in a bowl with an electric mixer until egg whites are foamy. Gradually beat in sugar, a few tablespoons at a time, beating until the sugar dissolves in the meringue before adding more. Continue beating until the meringue is glossy and forms a stiff peak when beaters are lifted straight up out of the bowl; beat in vanilla extract. Spoon the meringue into a pastry bag fitted with a small piping tip.
Pipe meringue into small bone shapes on the prepared trays. You must pipe all the shapes at once or the meringue will deflate.
Place meringue bones into the preheated oven and bake for 1 hour. Do not be tempted to open the oven door or peek during baking. Turn the oven off and leave the meringue bones to cool in the oven for 1 hour. Best not to open the door at this stage either.
Gently and carefully remove meringues from foil to prevent broken bones.