Ahead of time, soak corn husk in really hot water, set aside. They should soak for at least an hour.
In the stand mixer, whip together the butter and shortening until fluffy. Mix in the baking powder and salt. Gradually mix in the masa harina. Then gradually mix in the sweetened condensed milk, water and evaporated milk. Once incorporated really well, mix on med/high speed for 7-8 minutes or until masa looks light and fluffy.
Fold in 4 cups of previously prepared and chilled, arroz con leche. Reserve the remaining arroz con leche. If you like to add raisins into the masa, fold them in now. Or you can add a few raisins to the center of tamal once you spread the masa on corn husks.
Shake off excess water from corn husk. Using a large spoon add some masa to the center of the corn husk to resemble a square. Spoon in, down the center of masa some of the reserved arroz con leche. If adding a few raisins, add them near the arroz con leche. Fold in the sides, then fold down the flap.
Fill steamer pot with water to the rim at the bottom. Add steamer insert. Place a few extra corn husk to the bottom of steamer pot, Arrange tamales standing up, with the open side up. Cover tamales with extra corn husk and a clean kitchen towel. Cover with lid.
Place steamer pot on high heat, to start. After about 15-20 minutes when it begins to steam rapidly, lower heat to below medium. Set timer for 1 hour. Check one tamal after that time. Let it cool for a few minutes before attempting to open. If the husk peels away easily, then the tamales are done. If not, steam for another 15 minutes. Let them sit off the heat for 25 minutes more to finish setting up. Serve tamales on top of arroz con leche for a delicious dessert!