Defrost the banana leaves overnight in the refrigerator if frozen, and cut off any hard sections or sections with holes. Cut the leaves into unbroken 12-inch pieces. If the leaves are pliable, proceed with recipe. If not, steam the banana leaves in the top of a double boiler until soft and pliable, 20 to 30 minutes. Set aside until ready to assemble the tamales.
In a small saucepan, gently heat the rum. Place the raisins in a small bowl and cover with the warm rum. Let the raisins soak and absorb the rum while you prepare the masa filling.
In the bowl of an electric mixer beat the vegetable shortening until very light, about 1 minute. Add the sugar and half of the masa, and beat until combined. Mix the milk and coconut milk and add alternately with the remaining masa in several batches to the mixture until mixture is the consistency of medium-thick cake batter. Add baking powder, salt and cinnamon, and beat for 30 seconds. Add the melted butter and beat to incorporate, being careful not to overmix.
Spoon about 3 tablespoons of the masa batter into the center of a banana leaf. Place about 2 teaspoons of raisins in the center. Fold one side over the batter, then the other to enclose the filling. Bring the bottom and the top over the filling to create a small package. Tie the tamale with a piece of string, and repeat with the remaining masa batter and filling.
Line a steamer insert with a layer of unused banana leaves. Lay the tamales in the lined steamer, and steam over simmering water until cooked through and the tamales are cooked through and release easily from the banana leaf wrappers, about 1 to 1 1/2 hours.
Yield: 18 tamales, serving 6 to 8