1. Preheat oven to 325 degrees. Line 2 rimmed baking sheets with nonstick baking mats. Mix 1 cup sugar, the vanilla seeds, and 5 tablespoons butter. Slice 1/2 inch from both ends of pineapple, and discard. Cut 2 slices crosswise, 3/4 inch thick, and set aside. Peel the remaining pineapple, and slice into 3/4-inch-thick spears, working around core. Discard core.

2. Divide half the butter mixture between baking sheets, dropping it by rounded teaspoons onto mats. Arrange pineapple spears and rounds in a single layer on top of butter mixture. Top pineapple with spoonfuls of remaining butter mixture. Roast, rotating pans and basting with pan juices after 15 minutes, for 30 minutes. Flip pineapple, then roast until dark golden brown, about 30 minutes more. Let cool for 5 minutes. Cut rounds into 8 wedges, and reserve for garnish.

3. Meanwhile, bring reserved vanilla pod, cardamom, allspice berries, water, and remaining 1/4 cup sugar to a boil in a medium saucepan. Remove from heat, and let stand for 30 minutes. Strain, and discard solids. Working in batches, blend liquid and roasted-pineapple spears with their pan juices on low speed until coarsely pureed. Increase speed to high, and process until smooth, about 2 minutes. Strain through a fine sieve into a medium saucepan, pressing firmly on solids. Discard solids.

4. Heat pineapple mixture over low heat until heated through, then add rum if using. Divide among glasses, and top each with some of the remaining 2 tablespoons butter and reserved pineapple wedges.