Remove the leafy crown and slice the pineapple in half. Cut off the skin and eyes and cut into 8 wedges. Cut off the woody core, then push the wedges onto 8 soaked wooden skewers and set aside until needed.
For the syrup, put the brown sugar, water, vanilla, lime zest and a squeeze of juice into a small saucepan. Whisk until the sugar has dissolved and the vanilla seeds have dispersed. Bring to the boil, turn down the heat and simmer for 8-10 minutes, or until syrupy. Stir in rum, if using. Set aside until ready to serve.
When you’re ready to barbecue, sprinkle the pineapple wedges with caster sugar and barbecue for 4-5 minutes on each side, or until slightly charred. Serve with the syrup and some ice cream, if you like.