To a 10 oz glass add granulated sugar and mint leaves. Muddle with a muddler or a wooden spoon until the mint is sufficiently mashed and fragrant. To the muddled sugar/mint mixture add 1 piece of the quartered lime. Mash with the muddler to release some of its juice and zest. Add coconut cream, followed by both rums. Stir to combine. Add the 3 remaining lime pieces, ice, and top with seltzer. Garnish with a piece of sugar cane.