Step 1Grill the pineapple for 4 minutes on one cut side, then turn and grill for 4 minutes on the other cut side.
Step 2Divide the butter into four pieces. Cut the vanilla beans in half lengthwise. Place each section of pineapple on a large piece of aluminum foil and top with a piece of butter. Scrape the seeds from half a vanilla bean onto each, then place the pod on top. Fold over the foil and tightly crimp the edges to make sealed packets.
Step 3Combine the dried mint, dried rose petals and sugar in a small bowl and set aside.
Step 4Place each packet on the grill and barbecue with the cover closed and vents open for 8 minutes.
Step 5To serve, remove each pineapple piece from its packet, discarding the vanilla bean and foil. Drizzle with butter from the packet, then sprinkle each generously with the mint-rose-sugar mixture. Serve with a scoop of vanilla ice cream or frozen yogurt, if desired.