In a cocktail shaker filled with ice, combine the rum, allspice dram, pumpkin puree and lemon juice. Cover and shake vigorously. Strain into an ice-filled Collins glass, top with ginger beer and serve.
Allspice Dram is a liqueur flavored with allspice berries. It's also known as pimento dram. Find it in the liquor section of well-stocked liquor stores (most common brands are St. Elizabeth or The Bitter Truth Pimento Dram) or online.